The Legacy of Carbon Dioxide: Past and Present Impacts 9780367190804, 036719080X, 9780367191344, 0367191342

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The Legacy of Carbon Dioxide Past and Present Impacts

The Legacy of Carbon Dioxide Past and Present Impacts

Paul J. Karol Professor Emeritus of Chemistry Carnegie Mellon University

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2019 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed on acid-free paper International Standard Book Number-13: 978-0-367-19134-4 (Hardback) International Standard Book Number-13: 978-0-367-19080-4 (Paperback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged, please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.

Library of Congress Cataloging‑in‑Publication Data Names: Karol, Paul J., author. Title: The legacy of carbon dioxide : past and present impacts / Paul J. Karol. Description: Boca Raton : CRC Press, Taylor & Francis Group, 2019. | Includes bibliographical references. Identifiers: LCCN 2019004600| ISBN 9780367191344 (hardback : alk. paper) | ISBN 9780367190804 (pbk. : alk. paper) Subjects: LCSH: Atmospheric carbon dioxide. | Atmospheric chemistry. | Geochemistry. | Carbon dioxide. | Carbon--Isotopes. Classification: LCC QC879.8 .K37 2019 | DDC 546/.6812--dc23 LC record available at https://lccn.loc.gov/2019004600 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents Preface...............................................................................................................................................xi Acknowledgment............................................................................................................................ xiii Author............................................................................................................................................... xv Academic Genealogy......................................................................................................................xvii Chapter 1 Starting Elements..........................................................................................................1 Nucleosynthesis.............................................................................................................1 The Elements of Chemistry........................................................................................... 2 Atoms............................................................................................................................4 Compounds and Molecules...........................................................................................5 Chapter 2 Early Earth and Our Solar System................................................................................7 Gestation: Forming Planets with Atmospheres............................................................. 7 Early Atmospheric Activity...........................................................................................8 Atmospheric Influences during Earth’s Infancy........................................................... 9 Sibling Planets............................................................................................................. 11 The Toddler Earth....................................................................................................... 12 Adolescent Earth......................................................................................................... 13 Earth’s Puberty............................................................................................................ 14 Commencement........................................................................................................... 15 Chapter 3 Discovery..................................................................................................................... 17 Van Helmont................................................................................................................ 17 Hales............................................................................................................................ 18 Black............................................................................................................................ 18 Cavendish....................................................................................................................20 Hamilton...................................................................................................................... 21 Priestley....................................................................................................................... 21 Chapter 4 Structure...................................................................................................................... 23 CO2.............................................................................................................................. 23 Other Properties of Carbon Dioxide...........................................................................25 Other Carbon–Oxygen Molecules..............................................................................26 Phases..........................................................................................................................26 Chapter 5 Radiocarbon and Its Dioxide....................................................................................... 29 Cosmic Rays................................................................................................................ 29 Production of Radiocarbon......................................................................................... 29 Isotope Effects............................................................................................................. 30 Radioactive Decay of Carbon-14................................................................................. 31 Age Calibrations.......................................................................................................... 32 Deviates....................................................................................................................... 35 v

vi

Contents

Other Complications................................................................................................... 35 Anthropocene.............................................................................................................. 37 Modern Tweaks........................................................................................................... 39 Chapter 6 The Air Today............................................................................................................. 41 Air Pressure................................................................................................................. 41 Air Composition.......................................................................................................... 41 Carbon Dioxide Variations.......................................................................................... 43 Historical Prescience on Carbon Dioxide...................................................................46 Chapter 7 Ye Olde Aire................................................................................................................ 49 Mapping Time............................................................................................................. 49 History of the Atmosphere.......................................................................................... 51 Precambrian Air.......................................................................................................... 51 Phanerozoic................................................................................................................. 53 Early Paleozoic Era..................................................................................................... 54 Carboniferous Period.................................................................................................. 54 Mesozoic Era............................................................................................................... 55 Cenozoic Era............................................................................................................... 56 Chapter 8 Proxies......................................................................................................................... 61 Borate Proxy................................................................................................................ 61 Reading about the Atmosphere in Ice......................................................................... 62 Cave Droppings........................................................................................................... 67 Reading about the Atmosphere in Tea Leaves............................................................ 68 Soiled Records............................................................................................................. 69 Alkenones, Marine Algae Compounds....................................................................... 70 Reading about the Atmosphere in Amber................................................................... 71 Chapter 9 Fire.............................................................................................................................. 73 Forests......................................................................................................................... 73 Bogs............................................................................................................................. 75 Megafires.....................................................................................................................80 Global Conflagration...................................................................................................80 Coal Fires.................................................................................................................... 81 Chapter 10 Carbon Dioxide and Water.......................................................................................... 83 Water........................................................................................................................... 83 pH................................................................................................................................ 83 Buffers, Acidity, and Alkalinity..................................................................................84 Enter CO2.................................................................................................................... 86 CO2 Solubility............................................................................................................. 86 Temperature Effects.................................................................................................... 88 pH Effects.................................................................................................................... 89 Pressure Effects...........................................................................................................90 Carbon Dioxide and Seawater.....................................................................................90

Contents

vii

Restraint: The Revelle Factor......................................................................................92 Fresh Water Considerations.........................................................................................94 Chapter 11 Going with the Flow....................................................................................................97 Rivers and Oceans.......................................................................................................97 Water Carrier...............................................................................................................97 The Oceans................................................................................................................ 100 Feeding the Oceans................................................................................................... 103 Deep Sea Vents.......................................................................................................... 104 Chapter 12 Carbonates: The Enduring Legacy............................................................................ 107 Setting the Scene....................................................................................................... 107 Whither the Ocean’s Carbon Dioxide?...................................................................... 107 Chalk......................................................................................................................... 108 Solubility, Saturation, and Supersaturation............................................................... 109 Distribution................................................................................................................ 111 Carbonates in the Ocean........................................................................................... 112 Caves......................................................................................................................... 114 Cement....................................................................................................................... 115 Marble....................................................................................................................... 115 Alabaster................................................................................................................... 116 Travertine.................................................................................................................. 116 Ear Sand.................................................................................................................... 118 Other Carbonate Minerals......................................................................................... 119 The Enduring Legacy................................................................................................ 120 Chapter 13 Volcanoes.................................................................................................................. 121 Out of this Earth........................................................................................................ 121 Carbon Dioxide Powered Volcanoes......................................................................... 124 Killer Lakes............................................................................................................... 125 Flood Volcanism and Traps....................................................................................... 128 Dinosaurus Extinctus................................................................................................ 131 Underwater Flood...................................................................................................... 132 Earthquakes............................................................................................................... 133 Chapter 14 Photosynthesis........................................................................................................... 135 Significance............................................................................................................... 135 Discovery................................................................................................................... 136 Mechanism of the Synthesis...................................................................................... 138 Calvin Photosynthesis Cycle..................................................................................... 139 More Water, Less Water, Any Alternative Paths?..................................................... 142 Light.......................................................................................................................... 143 Changes..................................................................................................................... 145 Hidden Carbon Dioxide............................................................................................ 146 Night and Day, Summer and Winter, Up and Down................................................. 148 Photosynthesis Alternatives...................................................................................... 150 Nobel Prizes Related to Photosynthesis.................................................................... 151

viii

Contents

Chapter 15 Respiration and Metabolism..................................................................................... 153 Photorespiration........................................................................................................ 155 Respiration and Hemoglobin..................................................................................... 157 CO2 and Blood........................................................................................................... 159 Hyperventilation........................................................................................................ 161 It’s a Croc!................................................................................................................. 161 Apollo 13................................................................................................................... 162 Breath Analysis......................................................................................................... 164 Planned Unconsciousness......................................................................................... 164 Carbon Dioxide Flooding.......................................................................................... 164 Chapter 16 Weathering................................................................................................................ 165 Water on the Rocks................................................................................................... 165 CO2 Sponge............................................................................................................... 165 Discovery................................................................................................................... 166 Weathering and Carbon Dioxide............................................................................... 168 Early Earth................................................................................................................ 169 Life and Weathering.................................................................................................. 169 Yet Another Consideration........................................................................................ 171 More Recently........................................................................................................... 172 Uh-Oh, Sinkholes...................................................................................................... 172 Chapter 17 Carbon Dioxide and Ice Overs.................................................................................. 175 Snowball Earth.......................................................................................................... 175 Runaway Glaciation.................................................................................................. 180 Runaway Sauna......................................................................................................... 181 Survivors................................................................................................................... 183 Embedded Compasses............................................................................................... 184 Carbonate Isotope Effects......................................................................................... 184 Snowball Earth: Beta Version................................................................................... 186 Chapter 18 Food and Drink......................................................................................................... 187 Digestion and Carbon Dioxide.................................................................................. 187 Bread, Beer, Champagne........................................................................................... 190 Soda........................................................................................................................... 190 Chapter 19 Fossil Fuels................................................................................................................ 191 Origins....................................................................................................................... 191 Alternative Source..................................................................................................... 194 But Wait! There’s Mire.............................................................................................. 194 Chapter 20 Isotope Stories........................................................................................................... 197 Storytellers................................................................................................................ 197 Early Ice Ages, Briefly.............................................................................................. 197 Isotope Effects...........................................................................................................200 Why Any Effect?....................................................................................................... 201

Contents

ix

Isotope Fractionations in Nature...............................................................................202 Mass Measurements..................................................................................................202 Carbon Isotope Fractionations..................................................................................204 T Recs........................................................................................................................204 More Complex Influences on Isotope Fractionation.................................................206 “You Are What You Eat”..........................................................................................207 The Vostok Saga........................................................................................................208 Coordinated Chronicles............................................................................................. 210 The Devil’s Hole Tale................................................................................................ 211 Postscript................................................................................................................... 213 Appendix: On a Piece of Chalk................................................................................................... 215 Bibliography.................................................................................................................................. 229 Index............................................................................................................................................... 231

Preface The sphinx, Apollo 13, the White Cliffs of Dover, mass extinctions, stomach acid, and killer lakes in Africa: what do these all have in common? It is the legacy of carbon dioxide. Motivation to assemble the assorted roles of carbon dioxide in our lives and surroundings was a personal quest for the author. It was catalyzed by a desire to come to terms with one of today’s hot issues, pardon the pun: global climate change. Over the past years, I have read what proved to be countless scientific articles and media opinion pieces on global climate change and human influence on climate. Arguments from opposing sides could at times then seem equally convincing, which proved frustrating. I decided to educate myself further and found the topic and the paths down which my pursuit led me to be more and more fascinating. Yet I was not overcome with any obvious decision as to where the future of climate change would likely bring us. Mostly to put my thoughts in order and to try to complete my understanding of many related natural and human-­ generated ­phenomena, I have authored this book and titled it in as an unencumbered way as possible, The Legacy of Carbon Dioxide. The Oxford English Dictionary regards legacy as anything handed down by an ancestor or predecessor. For the sake of simplicity, I have taken certain liberties with terminology and with quantitative expressions. For example, I ignore the slight difference between weights expressed as tons or tonnes. I use % changes rather than the geophysicists’ preferred ‰. I apologize to the many experts in the field for being incomplete in my coverage and also, probably, error prone since the literature, overwhelming as it is, changes on almost a daily basis. My objective is to allow each reader to recognize how startlingly complex the entire issue of carbon dioxide is, how many different and unexpected ways carbon dioxide affects us, how we measure these effects, and how various are the degrees of certainty or uncertainty in what is currently understood. Trying to keep up to date with studies involving carbon dioxide has proven to be a challenge. Here, the annual publication rate of scientific articles with either “carbon dioxide” or “CO2” in their title is shown as a function of time. The points along the bottom flat curve correspond to scientific articles with “carbon monoxide” in their title for comparison, averaging a nearly constant four publications per year over the last half century. Data was extracted using the Google Scholar search engine.

xi

xii

Preface

I have assiduously eschewed what prompted my interest, modern human influence on global climate, hoping each reader will reach his or her own conclusion from an unbiased presentation; a worthy goal I think – one that spites the observation of a famous author. There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact. MARK TWAIN Study the past if you would define the future. CONFUCIUS Every effort has been made to keep this book lean so as to encourage its reading.

Acknowledgment One source of inspiration, advice, and encouragement and one source of invaluable assistance constitute this brief recognition of support. Meryl H. Karol, Professor Emerita of Environmental Toxicology at the University of Pittsburgh’s Graduate School of Public Health and my wife, best friend, and companion for over 50 years, read most of the manuscript in its various incarnations and made countless constructive suggestions. The Mellon Institute Library staff at Carnegie Mellon University provided priceless, expedient, and instrumental assistance in accessing scientific literature crucial to moving this project along and keeping it current. Without either of these resources, my mission was doomed. Thank you. PAUL J. KAROL

xiii

Author Paul J. Karol is a linear academic descendent of Joseph Black, the discoverer of carbon dioxide. Karol has been on the chemistry faculty at Carnegie Mellon University in Pittsburgh for over 40 years and has received two awards for teaching during that period. His undergraduate degree in chemistry was from Johns Hopkins University and his postdoctoral research was done at Brookhaven National Laboratory. His doctorate degree in nuclear chemistry was acquired at Columbia University under the auspices of Dr. J. M. Miller. Prof. Karol has served as Chair of the Division of Nuclear Chemistry and Technology of the American Chemical Society, as Chair of the Committee on Nomenclature, Terminology and Symbols of the American Chemical Society, as Chair of the Committee on Nuclear and Radiochemical Analysis of the International Union of Pure and Applied Chemistry, and as Chair of the Joint Working Party on the Discovery of New Elements of the International Union of Pure and Applied Chemistry and the International Union of Pure and Applied Physics. He served as Visiting Professor to the Institute of Nuclear Physics in Legnaro, Italy, and the Japan Atomic Energy Research Institute in Tokai, Japan.

xv

Academic Genealogy During my efforts in compiling this book, I learned that I am a direct academic descendant of Joseph Black, the person credited with the characterization, if not the discovery, of carbon dioxide in 1754. This academic genealogy is shown below. Julian M. Miller, Professor of Chemistry at Columbia University in New York City, did research in the field of nuclear chemistry. His doctoral advisor was Richard Dodson. Richard Dodson was the first chairman of the Chemistry Department at Brookhaven National Laboratory and received his PhD from Johns Hopkins University under the direction of Prof. Robert D. Fowler. Robert D. Fowler was on the faculty of Johns Hopkins University in Baltimore and was awarded his doctoral degree in chemistry from Willard at the University of Michigan. Hobart Hurd Willard from Erie, Pennsylvania, received his PhD in 1909 at Harvard. The title of his thesis was “A Revision of the Atomic Weights of Silver, Lithium, and Chlorine.” His studies focused on analytic methods and inorganic chemistry, particularly that of perchloric and periodic acids and their salts. Willard was a member of the chemistry faculty at the University of Michigan. His doctoral thesis advisor was T. W. Richards. Theodore William Richards was the first American to receive the Nobel Prize in Chemistry. Richards studied at Harvard, taking as his dissertation topic the determination of the atomic weight of oxygen relative to hydrogen. His doctoral advisor was Josiah Parsons Cooke. Richards returned to Harvard as an assistant in chemistry, then instructor, assistant professor, and finally full professor. Josiah Parsons Cooke from Boston, Massachusetts, a member of the National Academy of Sciences and president of the American Academy of Arts and Sciences published over 40 papers in his career. He received his AB in chemistry from Harvard in 1848. I. Bernard Cohen, the distinguished historian, describes Cooke as “the first university chemist to do truly distinguished work in the field of chemistry” in the United States. Cooke’s education at Harvard was under the mentorship of Benjamin Silliman, although it is reported that Cooke was largely self-taught. Benjamin Silliman, a chemist with a degree in law as well with a deep interest in geology and mineralogy, Silliman travelled extensively and became a professor at Yale despite no formal training in natural science. His influence by Thomas Hope led him to research in chemistry. Thomas Charles Hope was Professor of chemistry and medicine at Scotland’s University of Edinburgh. He was the discoverer of the element strontium in 1791. Among his students, besides Benjamin Silliman, was Charles Darwin. Hope was mentored by Joseph Black in Edinburgh. Joseph Black, Scottish physician and chemist, was Professor of chemistry at the University of Glasgow and then the University of Edinburgh. Besides his pioneering work on carbon dioxide, he also did some foundational work on thermodynamics and invented a precision analytical balance.

xvii

1

Starting Elements

The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. BERTRAND RUSSELL (The Philosophy of Logical Atomism) Without carbon dioxide, nature would be very different, unrecognizable. Chief among its roles is as feedstock for photosynthesis in the plant kingdom. Where did carbon dioxide originate? What better place to start? Let us go for a brief visit back to the “start” according to current scientific understanding.

NUCLEOSYNTHESIS As the current concept of the universe’s evolvement goes, about one second after the “big bang,” the expanding universe was peppered with elementary particles including protons, neutrons, and electrons. Seconds later, deuterium (heavy hydrogen) and helium formed by cascades of fusion encounters, first between protons, and then with fusion residues. After a few minutes, three-quarters of the universe’s matter* was hydrogen and the rest essentially helium. Traces of deuterium and other light species were present as well. From this simple mixture, the earliest stars slowly condensed through gravitation. The consequent gravitational heating within stars opened pathways to the production of heavier and heavier combinations of protons and neutrons and the elements associated with them. Despite the current age of the universe at more than some dozen billion years, its elemental composition is still 99% hydrogen and helium, implying that nucleosynthesis has not proceeded very far yet…on the average. Local variations are quite interesting though. From the Nobel Prize winning efforts of Alsatian-born, German-educated physicist Hans Bethe, we know much about the reactions that fuel the stars and about the concomitant production of heavier and heavier elements.

* Exclusive of dark matter.

1

2

The Legacy of Carbon Dioxide

The earliest stars could sustain the fusion reactions associated with their abundant hot hydrogen by a sequence of steps in which two protons could first meld together to form a deuterium nucleus (proton plus neutron) accompanied by the release of a positively charged electron (called a positron) and some energy. A nucleus consists of positively charged protons – the number of which determines the element’s identity – plus a number of neutral neutrons. The deuterium could then fuse with another proton to form a new nucleus with two protons and a neutron. The element with two protons in the nucleus is helium and this particular product with a total of three nuclear components is called helium-3. Its formation is accompanied by the release of still more energy. As the amount of helium-3 accumulates in the blazing hot mixture, the small probability of two helium-3s fusing becomes rather influential. Two helium-3s produce one helium-4, freeing two protons and a large burst of energy. Helium-4 has two protons in it, which is why it is helium. Its distinction from helium-3 is that helium-4 has two neutrons for a total of four component particles in contrast to the three for helium-3. Nuclei that have the same number of protons but different numbers of neutrons are called isotopes of the element with the given proton number. The net result of these nuclear reactions is that four protons end up producing one helium-4 plus some lighter particles and energy. Helium-4 is a particularly stable combination of particles. That stability accounts both for the enormous amount of energy released in 4He formation and for the reciprocal difficulty of breaking it apart. The stability is also the reason that it is the dominant reaction product present in the proton-rich material, the deuterium and helium-3 being present as minute components. As the star matures and hydrogen is consumed by fusion to form denser, heavier helium, gravitational collapse can resume, heating up the mixture with its significant component of helium-4. Under the right conditions, a helium-burning cycle may commence. Here, two 4He nuclei can fuse to form a 8Be nucleus. Beryllium-8 has four protons (making it, thereby, beryllium) and four neutrons. But 8Be happens to be a very unstable system. On its own, 8Be breaks back down to two helium4s in a very, very short time, less than a millionth of a billionth of a second. However, when there is abundant helium-4 nearby, it becomes possible to fuse the beryllium-8 with another helium-4 before the beryllium has split up. This second step in the helium-burning cycle produces carbon-12.

8

Be + 4 He →12 C

As the mixture continues to brew, carbon-12 will have the right conditions to coalesce with yet another of the surrounding helium-4s producing oxygen-16, a nucleus with eight protons and eight neutrons. That, in brief, is the origin of the carbon and oxygen atoms that will link up to make our carbon dioxide molecular combination. Carbon and oxygen are the third and fourth most abundant elements in the universe.

THE ELEMENTS OF CHEMISTRY Everyone has probably heard the expression “opposites attract.” This is rigorously the case with electrostatic charges. The positive nucleus has a strong attraction for negative particles which are present as electrons. If the number of electrons around the nucleus equals the number of protons in the nucleus, then the combination is electrically neutral since the amount of charge on a proton is equal to that on an electron, but just opposite in sign. The nucleus with its entourage of electrons forms a neutral species called an atom. An atom, by definition, is uncharged. An atom of carbon has a total of six electrons and an atom of oxygen has a total of eight electrons. The total mass of an atom is that of the nucleus plus that of the very light electrons, less an extremely small amount that is released when the electrons become bound about the nucleus. The atomic mass can be determined very precisely, relative to some standard, using modern instruments. The mass standard adopted by all scientists is carbon-12, whose atomic mass is arbitrarily, but very conveniently, set at exactly 12 “mass units,” u. Atomic masses of some important isotopes of both carbon and oxygen are listed in the table along with the percentage of the isotope found in naturally occurring terrestrial samples.

3

Starting Elements

Mass*

Abundance

Carbon-12

Isotope

12.0000000 u

98.90%

Carbon-13 Carbon-14 Oxygen-16 Oxygen-17 Oxygen-18

13.0033548 u 14.0032420 u 15.9949146 u 16.9991312 u 17.9991603 u

1.10% ~0% 99.762% 0.038% 0.200%

* 1 u = 1.6605402 × 10−27 kg

What those abundance percentages mean can be illustrated by referring to carbon. Approximately 99 out of every 100 carbon atoms you would find are carbon-12. On occasion, an atom of carbon-13 would be found, one per 100 carbons. Since natural carbon is a mixture of mass 12 and mass 13 isotopes in the percentages indicated, the average atomic mass* would be approximately 0.99 × (12) + 0.01 × (13.00) = 12.01. In the case of carbon, this differs from 12 by a very slight amount. But for other elements, the natural isotopic mixtures can vary significantly from whole numbers. Also, much less obvious now, the excellent precision with which the abundances and masses can be determined turns out to be an extraordinarily useful target for analysis in carbon dioxide studies – its legacy – as we will see in later chapters. The isotope carbon-14 is radioactive and is naturally occurring to the extent of 1.2 × 10 −10% in the atmosphere. It was discovered in 1934 by American physicist Franz Kurie of Yale University who bombarded nitrogen with neutrons which themselves had been discovered in 1932. Carbon-14 is frequently, but ambiguously, referred to as radiocarbon. These are not the only known isotopes of carbon and oxygen. A dozen more isotopes of each have been discovered. All are radioactive, most with very short lifetimes. The identity and lifetimes of these curiosities are indicated in the table.

Other Known Carbon Isotopes

Average Lifetime

Other Known Oxygen Isotopes

Average Lifetime

C-8

?

O-12

?

C-9 C-10 C-11 C-15 C-16 C-17 C-18 C-19 C-20 C-21 C-22

0.18 seconds 28 seconds 29 minutes 3.5 seconds 1.1 seconds 0.28 seconds 0.14 seconds 0.07 seconds 0.02 seconds Less than 30 nanoseconds 0.006 seconds

O-13 O-14 O-15 O-19 O-20 O-21 O-22 O-23 O-24 O-25 O-26 O-27 O-28

0.01 seconds 1.7 minutes 2.9 minutes 39 seconds 19 seconds 4.9 seconds 3.2 seconds 0.10 seconds 0.07 seconds ? 4.5 picoseconds Less than 260 nanoseconds Less than 100 nanoseconds

* Some literature refers to atomic mass as atomic weight. Strictly speaking, this is incorrect because weight is something you determine by weighing and depends on gravity. On a roller coaster ride, you can be briefly weightless. On the planet Jupiter, you would be very heavy. But your mass would remain the same. Different gravitational attraction would result in different weights for the same object. However, in discussing the isotopes and elements as we have done briefly here, everything is done relative to carbon-12 and so the values are referred to as relative atomic weights.

4

The Legacy of Carbon Dioxide

In the table, you can see some of the lifetimes have not been determined yet. Expectations are that a few more isotopes are yet to be discovered. C-11 and O-15 are both used in sophisticated medical positron emission tomography (PET) scans because their radiations are easily measured and the locations of the radiation-emitting atoms can be pinpointed with good geometrical resolution.

ATOMS An atom, more or less, is 100,000 times as large as its bare nucleus. Put in a more pictorial perspective, if the typical atomic nucleus were as large as the period (“.”) at the end of this sentence, then the entire atom with its collection of electrons would be the size of a football arena. Electrons are spread around the nearly point-like central positively charged nucleus. Detailed electron behavior is beyond the scope of our discussions and is also quite esoteric in nature. All we need concern ourselves with is that the electrons are assigned to shells whose average distance from the nucleus gets larger and larger as each new shell first receives electrons. The assignment of electrons, at least for the first 20 elements with atomic numbers (proton count) ranging from 1 to 20 is that • • • •

The first two electrons go in the first shell. The next eight electrons are assigned to a second shell. The next eight electrons are assigned to a third shell. The next electrons start a fourth shell.

When a shell has its full complement of electrons, it is referred to as a filled shell. The significance of a filled shell is that it has unusual stability, that is, resistance to change. Below is a table of the first 20 elements arranged in order of increasing atomic number starting with hydrogen (element 1) in the upper left and reading left-to-right to calcium (element 20). The arrangement shown here allows a visualization of the repeating physical and chemical properties of the elements associated with the number of electrons in the last shell being filled. For example, the closed shell electron arrangements associated with total electron numbers of 2, 2 + 8 and 2 + 8 + 8 correspond to the eighth column or family of elements, called noble gases, all of which are very unreactive chemically. In contrast, the seventh column elements, with electron arrangements of 2 + 7 and 2 + 8 + 7, are called the halogen elements or halides. For elements in the first column, on the left, the electrons are configured as 1, 2 + 1, 2 + 8 + 1, and 2 + 8 + 8 + 1. Except for the lightest of these, hydrogen, the group is called the alkali elements, having just a single electron in the last shell.

Starting Elements

5

This is an abbreviated version of the Periodic Table first articulated nearly completely by the Russian chemist, Dimitri Mendeleev, in 1869. Although understanding the causes of the periodicity in properties had to await the discovery of the electron, of x-rays, and of radioactivity, we observe here that the properties, including the reactions of the elements, are influenced almost entirely by the number of electrons in their last shell, the so-called valence (from Latin valentia = capacity) electrons. For the first column (or “family” or “group”), there is one valence electron; for the seventh column there are seven valence electrons; for the eighth column, there are eight valence electrons (except for helium).

COMPOUNDS AND MOLECULES Atoms, positively charged nuclei plus their surrounding electrons, are able to lose or gain electrons, thereby becoming ions. Atoms that lose electrons become positively charged ions. Atoms that gain electrons become negatively charged ions. With their opposite charges, a positively charged ion and a negatively charged ion attract each other and can form a combination called a compound. Table salt, for example, is basically sodium which has lost an electron and chlorine which has gained an electron. That is, Na+ and Cl−. Persons on low-salt diets sometimes use a salt alternative that is K+ and Cl−: potassium chloride. Both sodium and potassium (as well as the heavier alkali elements) very readily tend to lose their one valence electron in forming compounds. The reciprocal of this is that chlorine and the halogens have a tendency to gain an electron in forming compounds. A simplified explanation of this is that in doing so, both the alkali elements (column one) and the halogen elements (column seven) are left with closed shell electron arrangements which we noted earlier are extra stable. Since much of chemistry involves combinations of the elements depicted above where the closed shells have eight electrons, it is common to see reference made to an octet rule as an indication that changes seem to lead to the closed shell structure. Although chemistry, the science of such transformations, is actually much more complicated than this, the octet rule can be used to model an enormous amount of what transpires. A compound is a combination of atoms of at least two different elements. It is one kind of molecule. A molecule is a combination of at least two atoms, which need not necessarily be different. Carbon is unique in the variety of ways in which it can combine with other carbons and/or other elements. The majority of known compounds contain carbon. A very simple class of carbon compounds is the hydrocarbons, consisting of just the element carbon combined with the element hydrogen. Natural gas encompasses the C1 through C4 hydrocarbons (in which the subscript indicates the number of carbons with the number of hydrogens left unspecified); gasoline encompasses C6 through C10; diesel fuels C14 through C30; and lubricating oils C26 through C40. There is an incredible number of possible molecules made from carbon and hydrogen combinations. Just the C30 hydrocarbons with the maximum number of hydrogens – 62 it turns out – all having the formula C30H62, comprise over 4 billion different possibilities. Our attention will focus on the compound carbon dioxide in subsequent chapters. For now, very briefly, the carbon dioxide molecule has one carbon atom combined with two oxygen atoms. Carbon dioxide has an analog that is also a very common terrestrial substance and worth mentioning. In the abbreviated Periodic Table, you could note that just below carbon in the fourth column, as part of its chemical “family,” is the element silicon, symbol “Si.” This element is the basis of the huge semiconductor industry, one of the commercial centers of which is branded “Silicon Valley” in California. The combination of silicon with two oxygen atoms is the counterpart to carbon dioxide and is called silicon dioxide. But even though many properties of silicon and carbon are similar, the differences are sufficient such that SiO2 is vastly distinct from CO2. Silicon dioxide is the essential component of quartz and the major constituent of “sand.” Silicon is the second most abundant element in Earth’s crust. Its 26% abundance is exceeded only by that of oxygen. For many years, it was thought that the only combinations of atoms possible were those in which positive and negatively charged species were involved. Amadeo Avogadro adopted the seventeenth

6

The Legacy of Carbon Dioxide

century term “molecule” in 1811 to indicate combinations of identical species. He did this to explain some very simple experiments involving gases and the simple relationship among the volumes of combining gases and produced gases discovered by the Frenchman Gay-Lussac. It was Avogadro who proposed that the elemental form of oxygen consisted of two atoms in a molecule that we would now write as O2. Hydrogen gas was proposed to be H2. Avogadro’s hypothesis was resoundingly rejected by scientists at the time because there was no way to understand what would hold two oxygen atoms together since they were both neutral: uncharged. And there was also the unanswered question as to why just two atoms were involved. Why not three, or four, or more? These were extremely reasonable questions at the time, but were not evidence that the hypothesis was incorrect. The questions merely illustrated that the phenomenon of bonding in chemistry was not understood in the early 1800s. Patterns and trends can be misleading.

2

Early Earth and Our Solar System

The thinker makes a great mistake when he asks after cause and effect. They both together make up the indivisible phenomenon. GOETHE Before commencing the detailed drama of carbon dioxide, we must continue to set the stage: our planet Earth. A quick look at our solar system and some ideas about it will serve this purpose well. Omitted from the discussion is the limited degree of confidence associated with some of the descriptions. This chapter should be peppered with “maybe,” “perhaps,” “it is thought,” and other qualifying expressions. The concepts are all reasonable, though, and science has continuously adjusted its viewpoint as more and more sophisticated studies are done.

GESTATION: FORMING PLANETS WITH ATMOSPHERES Based on analysis of meteorites – solid debris from outer space – many astronomers are convinced that a supernova explosion preceded the formation of our sun when the nearly 14-billion-year-old universe was two-thirds of its present size. The birth of the solar system is dated to just under 5 billion years ago. Gravitational condensation of a massive cloud of dust and gas whose composition was thought to be like that of the sun formed around the solar core. From this contraction, we got the sun and the planetary disk. The first person known to have suggested that a nebular origin led to the solar system was the Scot-descended German philosopher, Immanuel Kant. Following his telescope observation of spiral nebulae, Kant wrote in detail his theory of the origin of the sun and the planets in Universal Natural History and Theory of Heaven in 1755. But the publisher went bankrupt, leaving the book essentially unknown for decades. As the solar nebula contracted and heated up, the volatile elements were driven to the outer portion of the planetary disc that was taking shape. The inner planets that formed close to the massive sun were correspondingly enriched in heavier elements. Recently, evidence using the slow radioactive decay of 238U (uranium is element 92) into Pb (lead, element 82) implies that Earth as a planet may have accreted as early as 30 million years after the formation of the solar nebula. And even more recent studies suggest this might have taken as little as 3 million years to be accomplished. Metals would sink to form the core and silicates rise to form the crust rapidly, taking perhaps 1 million years.* The initial atmospheres of the inner planets, including Earth, should have been mutually similar. The outer planets were endowed with light elements. For the inner planets, volatile substances, those that are gases at warm to moderately high temperatures, were not incorporated into planet formation for the most part. Exceptions would be those substances, oxygen and carbon dioxide for instance, that could chemically react to form nonvolatile solids such as oxides and carbonates. Volatility, on the other hand, accounts for the rarity on Earth of the noble gases, the eighth group in the Periodic Table containing helium, neon, argon and more. Yet, in seeming contradiction, helium is modestly available (for balloons…and research) and argon is the third most abundant gas in our atmosphere now. Helium and argon are two anomalies whose presence is well known to be due to radioactive decay. In the case of helium, its occurrence is a consequence of the radioactive decay in Earth’s core of uranium- and thorium-containing * T. S. Kruijer et al., “Protracted core formation and rapid accretion of protoplanets,” Science 344, 1150–1154 (2014).

7

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The Legacy of Carbon Dioxide

substances, both of which are alpha-particle emitters. The alpha-particle is identical to a helium atom which lacks its two electrons at emission but quickly picks them up. As far as argon is concerned, its origin is in the potassium (K) in Earth’s crust. Potassium, the 19th element, consists naturally of three isotopes, mostly 39K, but with about 6.7% 41K and 0.012% 40K. The last is radioactive and decays most of the time to 40Ar, a stable isotope of the 18th element. Almost all the argon in the atmosphere is 40Ar and owes its abundance to the decay of 40K at a rate determined by the 40K halflife of 1.4 billion years (or average lifetime of 2 billion years). The amount of 36Ar, another stable isotope of argon, is a million times more abundant than 40Ar in the sun than in Earth’s atmosphere. That anomaly is consistent with the total loss of all volatile primal argon during the initial formation of our planet. Similarly, the Mars Rover in 2013 determined that argon in the Martian atmosphere is 99.92% 40Ar.* Argon isotope abundance is part of the evidence that Earth did not retain its primary (original) atmosphere, for if it had, the relative abundances of the various argon isotopes would have been much more solar-like. The comparison below shows solar system abundances of some elements alongside terrestrial abundances, all adjusted in comparison to an arbitrary benchmark of “10,000” for silicon, a very abundant element in both domains.

Element

Solar System Abundance Relative to Si = 10,000

Terrestrial Abundance Relative to Si = 10,000

% Retained Terrestrially Relative to Si

H

350,000,000

84

0.000024

He C N Ne Na Al Si

35,000,000 80,000 160,000 50,000 462 882 10,000

0.00000035 71 21 0.0000012 460 940 10,000

0.000000000001 0.09 0.013 0.0000000024 ≈100 ≈100

The difference in relative abundances for the volatile elements is striking, especially for the noble gases helium (He) and neon (Ne). For all intents and purposes, these form no stable and no nonvolatile compounds. Their minor presence may be accounted for by recognizing that trace quantities do adhere to surfaces like those of dust particles. Compared to the volatiles, the last elements tabulated above manifest the opposite behavior. Being extremely nonvolatile when in compounds, as is usually their situation, they are referred to as refractory and, like most of the other refractory substances (not shown), have relative abundances that likely mirror those in the primary nebular cloud from which the solar system arose.

EARLY ATMOSPHERIC ACTIVITY There is little surviving evidence of the early accretion scenario, that is, the first half-billion years. But there are models. One model has a “blowoff” commencing about 50 million years after the sun finished contracting. The “blowoff” is a rapid, hydrodynamic outflow of mostly hydrogen that, like a swift wind, carries along other, heavier gases. In 2003, the blowoff process was observed on an extrasolar planet – Osiris – in the constellation Pegasus some 150 light years away. The amount of * P. R. Mahaffy et al., ”Abundance and isotopic composition of gases in the Martian atmosphere from the Curiosity rover,” Science 341, 263–266 (2013).

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9

hydrogen required for the blowoff model amounts to some 88 oceans worth of hydrogen, that is, from the hydrogen in that much water. This seems like a huge amount, but the large, outer planets of our solar system are known to still contain ~50% ice. Yet since hydrogen is a volatile gas, it should no longer have been present by the time Earth’s accretion concluded. Nevertheless, the hydrogen could have arisen externally, from acquired gas or even from water, the latter being broken apart by energetic light hitting the atmosphere in a well-known process called photolysis. Moreover, frequent meteor and comet impacts on the cooled but young planet could easily lead to accrual of materials just like those that had been previously purged. Photographs of Earth’s atmosphere taken from the moon and filtered through wavelengths (“colors,” in a sense) specific to hydrogen atoms unequivocally show the presence of the gas hydrogen. Helium is exceptionally low in abundance in the atmosphere despite its constant production by alpha-decaying radioactive isotopes in Earth’s crust and mantle. The light gas escapes from the atmosphere because of the speed with which it is moving when at the temperature of the surroundings. But this obvious mechanism of thermal escape of light gases from a hot early Earth is no longer the favored picture for the loss of volatiles (although it could still be invoked for planetesimals – small protoplanets – as they accrete). It appears, though, that there is as yet no way to distinguish these volatile loss mechanisms from an alternative picture in which there was incomplete condensation of the initial nebular gas during planet formation.*

ATMOSPHERIC INFLUENCES DURING EARTH’S INFANCY In the absence of any atmosphere, overall temperature balance on a planet is due mostly to input (radiation from the sun, also known as insolation) minus reflection (albedo†). Using Mars’s measured albedo and assuming that solar luminosity has been constant (so as to simplify the estimation), Earth’s primitive surface temperature is calculated to have been about 260 kelvin (−13°C or 9°F). Reflected radiation portions can exceed 80% from thick clouds or from uncompacted snow. Water bodies have albedos around 10% and vegetated areas less than 20%. An early scene, startlingly, would be one of an essentially airless Earth. And frigid. A clever way of estimating what portion of the present atmosphere might be remnants of a primary atmosphere is to use the abundance of the gas neon as a yardstick. That is, use the current, relatively low amount of neon in the air as representative of the original atmosphere still around, that is, as a proxy. Nitrogen and oxygen gas levels relative to neon should be similarly low. For example, the previous table shows nitrogen in the sun to be twice the concentration of neon in the sun. But today, nitrogen is millions of times more abundant in air than neon. The reasonable interpretation is that only an extremely small fraction of the current atmosphere, mostly nitrogen, is original. And that comparison value is a significant overestimate because the neon now in the atmosphere was probably trapped in Earth’s interior eons ago rather than being in the original air itself. Neon is a problematic proxy and turns out not to help in confidently pinning down ancient air contribution. The history of the planet’s physical and chemical evolution is fascinating on its own merits and constantly being studied with improving technology. Our very brief review here does it no justice. Many scientists believe that much of the water of the oceans and of the gases in the early atmosphere was deposited here by comets and/or meteors and/or asteroids. This hypothesis is now being scrutinized through flybys of comets exploring relevant isotope ratios in comparison to those on Earth. The accretion of carbon for carbon dioxide is somewhat of a puzzle though. This is because the simplest carbon compounds such as carbon dioxide, carbon monoxide, and methane (CH4) won’t exist as ices at times of accretion of the inner (warm) planets. They are volatile and would be lost as vapors. However, heavier hydrocarbons are a possible source of carbon accretion and could * A. N. Halliday, “The origin and earliest history of the earth,” Treatise on Geochemistry, 2nd edition, 1, 149–211 (2014). † From Latin, albus, white.

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The Legacy of Carbon Dioxide

subsequently react chemically with water to produce carbon dioxide and hydrogen, the latter then most likely escaping the atmosphere. Truly, early planet structures were presumably high temperature, molten bodies. The high temperatures were due to energy released as the planetary masses collapsed under their own gravity; to energy deposited from bombardments by massive meteors abundant during the early age of the solar system; and to radioactive heating greater than four times today’s rate, due to the much greater abundance of not-yet-decayed but short-lived unstable elements. (Among these is a hafnium isotope, 182Hf, of element 72 with a 9-million-year half-life and which will be discussed below.) Evidence of the meteor impacts is easily recognized by looking at the crater-scarred surface of the moon and the planet Mercury. At such high temperatures, not only was outgassing probable, but the force of the exiting vapors’ blowouts could carry away lingering atmospheric gases much like a strong wind can carry pollution and light debris along with it. Gravitational collapse as a source of heat likely would have lasted only for some millions of years. As our planet matured, decreased frequency of impacts and slowing gravitational collapse allowed cooling to commence. Evidence based on noble gas isotope data implies that the early atmosphere’s mass was much greater than it has been in recent eons. Such blanketing would have allowed surface temperatures to be several thousand degrees, enabling the existence of oceans of magma.* A stable crust of less dense solids such as silicates would form as planet cooled. Some elements such as hafnium are more soluble in the molten silicates than in the melted core whereas some elements, such as number 73, tungsten (W) are more soluble in the core. Separation between these two would then occur. Studies of crustal 182W, the isotopic product of 182Hf decay,† indicate core formation occurred on the order of only 1 million years after planetary accretion, followed by silicates floating up to form the surface crust. Thermal energy release by volcanic activity would be the major cooling phenomenon. Volcanic outgassing gave rise to what we could call our secondary atmosphere: methane, nitrogen, hydrogen, ammonia, water vapor, carbon monoxide, and carbon dioxide. The sequence of volcanic gas abundance is believed to be that just listed, decreasing from methane. Ultraviolet light (UV), with energy high enough to disrupt chemical bonds, would arrive from the sun. The UV would break down some of the hydrogen-containing molecules, releasing hydrogen atoms. Owing to their lightness, H atoms would escape the gravitational pull of the planet and leave the atmosphere, though the intense bombardment by extraterrestrial bodies during these early (millions of) years was an auxiliary route to replenished volatile substances. In a relatively short time, the atmospheric composition would change substantially, becoming mostly carbon dioxide and nitrogen. This is also the case for the planets Venus and Mars. And still is, for them, as indicated below with their distances from the sun in parentheses; however, it is no longer so for our home planet.

Venus (67 Million Miles)

Earth (93 Million Miles)

Surface Temperature

745 K

280 K

225 K

Atmospheric Pressure CO2 N2 O2 Ar

90 atm 96.5% 3.5% 0.003% 0.003%

1 atm 0.035% 78% 21% 0.9%

0.01 atm 96.0% 1.89% 0.145% 1.93%

* A. N. Halliday, op. cit. † T. S. Kruijer et al., Science 344, 1150 (2014). ‡ P. R. Mahaffy et al., op. cit.

Mars (141 Million Miles)‡

Early Earth and Our Solar System

11

Although hydrogen escapes Earth’s atmosphere, it can be regenerated by chemical reactions. The reaction of water vapor with light of sufficiently short wavelength and therefore high enough energy is still occurring today as evidenced by x-ray photographs of Earth taken in 1972 by Apollo 16 from the moon which filtered the exposure so that only emissions from atomic hydrogen appear.

SIBLING PLANETS Early Venus probably had as much water vapor and/or liquid water as Earth. But this water was lost due to a “runaway greenhouse effect.” Being closer to the sun, Venus receives about twice the solar heat that Earth receives. The humidity must have been huge due to evaporation of any liquid water into the atmosphere. Water vapor, like carbon dioxide, is an effective greenhouse gas. With vast amounts of H2O in the atmosphere, decomposition of water by ultraviolet light would release hydrogen as atoms which would escape the atmosphere or combine with other atoms, perhaps with another hydrogen to form molecular H2, which is still light enough to escape. The present temperature on Venus is very high at 745 kelvin, as the above table shows. Obviously, in such heat there would not be rainfall,* nor would there be photosynthetic life. Since rain provides a means for weathering rocks (Chapter 16), and photosynthesis (Chapter 14) consumes CO2, both very effective processes on Earth for removal of carbon dioxide from the atmosphere, Venus’s CO2 has not been reduced from its high primordial level. Mars, being further from the sun, is at a lower surface temperature than Venus (and Earth). Condensation of water vapor occurs, but as ice and snow. It is estimated that Mars originally had perhaps 30 atmospheres worth of water condensed as evidenced (but not proven) by stream beds that are observed and from geological knowledge of how such remnants must have been formed. The average temperature was not higher than 220 kelvin (−50°C or −58°F). Both warmer and colder regions would exist too. When the carbon dioxide pressure got high enough, CO2 ice could form in the colder, polar regions. Although the relative abundance of carbon dioxide on Mars is high at 95%, the total amount is low since the air pressure, a measure of the total amount of atmosphere, is 1% of the atmospheric pressure on Earth. Calculations suggest that the formation of carbonate minerals (Chapter 12) has served to store as much Martian carbon dioxide as is the case on Earth, although not for all the same reasons. The temperature versus vapor pressure diagram here for water emphasizes the differences among the three planets. The solid curved line rising from the lower left to the upper right delineates vapor-condensation equilibrium where vapor and liquid or solid co-exist. (Liquid and solid are the “condensed” phases.) It indicates the conditions of temperature and pressure at which water vapor condenses to either liquid water (rain) or solid water (snow, ice). Where that curve hits the right edge of the figure frame corresponding to a water vapor pressure of one atmosphere on the bottom axis is the normal boiling point of water, 373 K (100°C or 212°F) where the liquid and gas are both present. The horizontal line separating liquid from ice on the right is at the normal freezing temperature: 273 kelvin (0°C or 32°F). It intersects the curved solid line at the triple point where vapor, liquid, and solid all naturally can co-exist. That is the only condition under which the three can be present at equilibrium. Those solid lines are thermodynamically fixed properties of water and independent of environment.

* The surface temperature of Venus may have been so high (647 K, the “critical point”) that the liquid form of water thermodynamically did not exist.

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The Legacy of Carbon Dioxide

Starting on the left of the figure near the bottom, the flat dashed line indicates the surface temperature of Mars as 220 kelvin (−50°C). If water vapor pressure at that temperature grew from near zero (10 −6 atm on the bottom axis) to the square data point at about 0.00002 atmospheres, any additional water vapor from yet higher pressure moving to the right on the figure would condense out as ice (or snow), since the vapor/ice equilibrium curve would be crossed (at the black square). Where the dashed line at flat pressure would be extended, no gaseous water would exist on the toocold Martian surface. In contrast, Venus starts out much warmer, at about 315 kelvin. Following the upper, long-dashed line, as the vapor pressure – the amount of water in the atmosphere – increases, the greenhouse effect begins to set in above 0.001 atmospheres. (That value is for water vapor pressure, not total atmospheric pressure.) The long-dashed line abruptly curves upward showing that the surface temperature on Venus would rise rapidly as the water vapor content of the atmosphere continued to grow, powering the greenhouse warming. If additional water vapor accumulates, the vapor would never cross either the vapor/ice equilibrium curve nor the vapor/liquid equilibrium curve. Venus would have neither ice nor liquid water. Earth sits delicately between the Venus/Mars extremes. With increasing amounts of water vapor, the temperature would remain constant until the greenhouse effect emerges above a few thousandths of an atmosphere of water vapor pressure, indicated by the dotted line in the figure. The temperature rises as more water vapor is added, but not as rapidly as the condensation equilibrium curve (solid curve) is rising with vapor pressure. At just above the “triple point” for water (where vapor, liquid, and ice can all three co-exist in equilibrium), the amount of water vapor in the atmosphere hits the maximum possible at that temperature – saturates – and condenses into liquid at the filled circle in the figure at 280 kelvin (7°C or 45°F) – rain.

THE TODDLER EARTH Earth’s early atmospheric chemicals were continuously resupplied by accretion from then-abundant comets and meteors. Water in the atmosphere is represented at this ancient stage by the left portion of the horizontal dotted Earth line in the figure. As the water vapor pressure in Earth’s atmosphere increases from about 0.01% of the present pressure, greenhouse warming begins and the temperature rises above that of the ice–liquid transition temperature. Liquid condenses from the atmospheric vapor, that is, it rains. Additional water accretion from extraterrestrial sources enables more and more rain, leading to river formation and oceans, but not to more atmospheric water vapor pressure. A fixed water vapor pressure is a fixed amount, not a fixed percentage: you don’t get absolute humidity above 100%.

Early Earth and Our Solar System

13

Earth’s atmosphere, notable by comparison to the other two planets, is different…fortunately. That Earth’s temperature fell in the range of liquid water rather than vapor (Venus) or ice (Mars) allowed for an additional pathway to redesign Earth’s surface. We’ll get to that later. However, it is well recognized that the sun was roughly a quarter less luminous during these early ages; enough so, that if this were the sole consideration, the planet should have been covered with ice for 2 billion years, nullifying the existence of liquid water and even life for which there is evidence after the first billion years. That expectation is countered by the known effects of heavy concentrations of greenhouse gases in the atmosphere. Many uncertainties pertain to this early era and interpretations are very model dependent.* Among the visual evidence for a wet environment are mud cracks and ripple marks in sediments indicating the presence of liquid water, confidently dated by radioactivity techniques to layers older than 3 billion years. Earth’s atmosphere has constrained temperatures to be moderately warm at a very roughly constant level, except for arguable periods in antiquity when extreme ice ages pertained. A fascinating state of affairs in its own right, ice-encrusted Earth and the role of carbon dioxide will be discussed in Chapter 17.

ADOLESCENT EARTH Besides atmospheric changes, there are alterations in the solid structures of the planet’s surface. Asteroid impacts during early stages of the planet’s history are presumed to have been of such intensity and size as to evaporate oceans to 3 km depths or more and to sterilize the surface repeatedly. Much of Earth’s earliest surviving crust dates back 3.8 billion years when the heavy asteroid bombardment likely faded (based on studies of lunar craters). Yet, the mineral known as zircon, highly resistant to weathering, transport, re-deposition, and erosion, has been used to establish a record longevity for the oldest crustal rock at 4.374 billion years.† Because of such initially hostile environmental conditions, the first 700 million years of Earth’s existence are referred to as the Hadean Period (after Hades). Some estimates suggest that during the Hadean, as many as 15 objects at least 100 miles wide struck Earth. Even moon-sized objects probably swept close by, driving major changes in the atmosphere. Complex life forms seem only to have arisen permanently several hundred thousand years later.‡ Geographic variation usually occurs extremely slowly, roughly measuring an average of a millimeter per year: up, down, sideways, whatever. Of course, there are exceptions at extremes. Millions of years of moving and colliding geologic plates raise mountain ranges. Rain, wind, and chemistry reshape and recycle the mountains into clay, silt, and dissolved substances that find their way eventually into the oceans, often depositing onto ocean bottoms. Earth may be somewhat unique in that its solid crust is continuously being destroyed, churned and regurgitated: a massive natural recycling scheme. Despite the sluggishness with which these processes typically transpire, there are occasions in which events take place with explosive rapidity. Besides conspicuous volcanic activity, there are asteroids, meteors, and comets that smash into Earth at thousands of miles per hour. Impacts can melt rock – even vaporize it. Shockwaves can shatter crystal structures far away and can demagnetize some rocks. Among the best known of these phenomenal episodes involves Arizona’s Meteor Crater, blown out perhaps 50,000 years ago. The crater is 1.2 km in diameter and 200 m deep. Agreement is that the perpetrator of this landmark was an iron–nickel meteorite perhaps 45 m in diameter, half the length of a soccer field. That is roughly the size of the small whitish area in the picture below at the center of the crater bottom.

* G. Feulner, “The faint young sun problem,” Rev. Geophys. 50, RG2006 (2012). † J. Wraltey et al., Nat. Geosci. 7, 219 (2014). ‡ There is a report on a “potentially biogenic carbon” in a 4.10 ± 0.01 billion-year-old zircon from Western Australia published by E. A. Bell, P. Boehnke, T. M. Harrison, and W. L. Mao in Proc. Natl. Acad. Sci. 112, 14518 (2015).

14

The Legacy of Carbon Dioxide

EARTH’S PUBERTY Only a half-century ago, scientific experiments suggested the origin of life, some 3.5–4 billion years ago, could be ascribed to reactions involving methane and ammonia in the atmosphere. The latter would be what’s called a chemically “reducing” atmosphere, the opposite of an oxidizing atmosphere that we now have. Electric discharge experiments (lightning in the laboratory) by Stanley Miller, working with Harold Urey in the 1950s, illustrated the formation of complex mixtures of organic substances, some of which were amino acids, arguably portending the possibility of life. Accumulated organic molecules in the early ocean were referred to as the primordial soup. In the early nineteenth century, Russian Alexander Oparin, followed shortly thereafter by Englishman J. B. S. Haldane, proposed such a scheme for the origin of life. They recognized also that oxygen could not have been present because it would have chemically detoured the production of many of the soup’s ingredients. But such an atmospheric recipe was not supported by further studies in the 1960s. Very recently, unpublished results from Miller’s laboratory showed that he also used an apparatus that mimicked a volcanic eruption accompanied by lightning. Miller also included hydrogen sulfide (H2S) in the gas mixture. The new analyses revealed that over 40 different amino acids and amines were generated, suggesting volcanic island arc systems could have provided an environment for some of the processes thought to be involved in chemical evolution and the origin of life. On the other hand, it was realized that Miller’s production of amino acids was probably not the critical initiation step for the origin of life because amino acids and amines are not self-replicating, a key characteristic of living systems. Also, recently, it was hypothesized and soon more widely accepted from indirect evidence that the early atmospheric methane and ammonia gas abundances would have been scarce. Carbon dioxide would have been plentiful. At first glance, the production of organic substances is much less apt to occur in such an atmosphere – bad news. However, a group of planetary scientists led by Feng Tian from Colorado proposed in 2005 that, contrary to the conventional picture, hydrogen would have been a major constituent of the young Earth atmosphere, comprising possibly as much as 30% of the total gas composition.* In this vision, the missing ingredient for synthesis of organic matter by electrical discharges in the atmosphere is re-established. Contradicting earlier mentioned considerations of the atmosphere’s composition, Tian and co-workers argued that hydrogen supplied by volcanic outgassing was not as efficiently able to escape the planet as previously predicted. There do exist competing scientific ideas for life’s launching, one divorced from the atmosphere. In the ancient oceans, sulfate (the highly oxidized form of sulfur, element 16), would have been prevalent. The abundance of sulfurous emissions from submarine hydrothermal vents would be ideal * F. Tian, O. B. Toon, A. A. Pavlov, and H. DeSterck, “A hydrogen-rich early earth atmosphere,” Science 308, 1014–1017 (2005).

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15

for organic synthetic reactions leading to complex structures, potentially with biological relevance. Such a model is now seriously considered as an alternative to the atmospheric source concept. And in 1970, analysis of the 100 kg Murchison meteorite revealed the presence of extraterrestrial amino acids suggesting their syntheses occurred during the early history of the solar system. The Murchison meteorite was an important finding since its 1969 descent and touchdown outside Melbourne, Australia, was widely observed and the meteorite was recovered and analyzed very quickly, minimizing the chance of contamination. Ultimately, all the different pathways may have contributed in different degrees to the characteristics of young Earth.

COMMENCEMENT All the processes occurring over Earth’s history continue to shape the carbon dioxide narrative: volcanism, erosion, photosynthesis, precipitation, subduction, weathering, combustion, decay, and evaporation. Let the legacy story commence.

3

Discovery

A man should look for what is, and not for what he thinks should be. EINSTEIN It is ironic that something as ubiquitous as carbon dioxide, a substance that certainly has been around at least since early Earth, wasn’t really noticed until a few hundred years ago.

VAN HELMONT The Belgian chemist and physician Jan Baptista van Helmont was likely the first to recognize that what we call air was not the only gaseous substance in existence. At the end of the sixteenth century and for yet another 150 years, the term air meant gas. Van Helmont, who lived from 1580 to 1644, recognized that the substance given off by burning charcoal was identical to that yielded by the fermentation of grapes. Van Helmont invented the word “gas” to describe the spiritus sylvestre (spirit of wood) emanating from the burning charcoal. The word derives from the Greek word “chaos” (χάος), although alternative, but erroneous, etymologies suggest the word derives from the Dutch word geest akin to the German geist meaning “spirit” or “ghost.” The Oxford English Dictionary has a 1662 quoted translation of Helmont’s work that reads: Because the water which is brought into a vapour by cold, is of another condition, than a vapour raised by heat: therefore…for want of a name, I have called that vapour, Gas, being not far severed from the Chaos of the Auntients. Gas is a far more subtile or fine thing than a vapour, mist, or distilled Oylinesses, although as yet, it be many times thicker than Air. But Gas it self, materially taken, is water as yet masked with the Ferment of composed Bodies.

Jan Baptista van Helmont

The last sentence in the quote is certainly strange. Van Helmont regarded a gas to be contained in all bodies as an ultra-rarefied condition of water. Van Helmont was a believer in water being a basic element of the universe, as were the ancient Greeks and many thereafter. Quantitative experiments were relatively new in his time, but he recognized their utility, even though many philosophers 17

18

The Legacy of Carbon Dioxide

frowned on the lowly concept of actually making measurements. His most noteworthy quantitative evaluation was growing a tree in a weighed amount of soil, nurturing it with weighed quantities of water and noting that the 164-pound-increased weight of the tree, taking into account fallen leaves, did not come from the soil but rather, in his view, from the added water. Carbon dioxide, of which van Helmont himself had become aware, was overlooked as a required nutrient.

HALES A half-century after van Helmont, the English botanist and physiologist Stephen Hales advanced methods for studying different gases by inventing a device to collect gases over water. Hales originally studied theology at Cambridge but was active in science on the side. Among his accomplishments were measurements on the growth of plants and the pressure of sap. Hales, too, was the first to measure blood pressure. Also, he made a significant contribution to public health with studies on the beneficial effects of fresh air ventilation. Hales became aware that some air contributed to the growth of plants, thus modifying van Helmont‘s misinterpreted weight gain experiments from the previous century. His 376-page work was published as Vegetable Staticks in 1727. Staticks was the science of weights and measures at the time.

Stephen Hales

BLACK Joseph Black, one of 15 children, was born in Bordeaux, France, on April 16, 1728. His father was a wine merchant of Scottish descent and his mother was from a Scottish wine trade family. He was sent by his parents first to Belfast and subsequently to Glasgow to study “arts,” that is, Latin and Greek. A few years later, he was convinced to take up a more marketable venture and chose medicine. At Glasgow University, his professor of medicine had just established lectures in the field of chemistry, at that time regarded as a sub-discipline of medicine. Black’s mentor was noted for encouraging students to engage in independent experiments. The medical professor, William Cullen, hired the 20-year-old Black as an assistant in the laboratory. Subsequently, Black pursued further medical studies in Edinburgh after which he returned to Glasgow as a Lecturer in ­chemistry. During the early Glasgow years, he began research for his MD thesis on the chemistry of magnesia alba, which means “white magnesia.” This is a form of hydrated magnesium carbonate sometimes

Discovery

19

called mild magnesian earth. Named after the ancient city in Asia Minor, Magnesia (in western Turkey, southwest of Smyrna), it has the formula 4MgCO3.Mg(OH)2.5H2O. Suspensions of magnesia alba are still used as milk of magnesia. Black’s 1754 dissertation was titled “De humore acido a cibis orto, et magnesia alba” (On the Acid Humour Arising from Food, and Magnesia Alba). The medical dissertation, written in Latin as was the tradition at the time, dealt mostly with the use of magnesia as an antacid. But it also included the discovery of carbon dioxide. An English translation was not made available for over 150 years despite the importance his discovery of carbon dioxide had in the field of chemistry. I have made no few experiments: many of them were new, and some even worthy enough of record: I therefore thought that it would not be unpleasing to those of my fellow students who are fond of chemical philosophy if I were to publish the more remarkable of them: and so I hope that they will be favourably received by such. For the result, whatever it be, of a new experiment is not to be neglected; since the foundation of chemical science, so useful to medicine, and yet still so very imperfect, rests on experiments alone.

The experiments, which led to the discovery of what Joseph Black termed “fixed air,” involved for the first time very careful weight measurements of magnesia alba as it was heated (to release carbon dioxide as the result of thermal decomposition). He followed the reactions of the products with acids or alkalis. Black observed that magnesia “is dissolved quickly and completely by vitriolic acid, and in their union a great abundance of air is expelled from them.” Vitriolic acid is the old name for sulfuric acid. The same occurred with acid of nitre (nitric acid) and distilled vinegar (acetic acid). Remember, at the time, the term “air” meant any “gas” in general. When 3 ounces of magnesia were heated to very high temperatures – a process called calcining – the solid became red hot. Allowed to cool, the solid residue lost more than half its weight, ultimately measuring “an ounce, three drams, and thirty grains.” A dram is one-eighth of an ounce and there are 60 grains to a dram. Only a small portion of the volatiles driven off was water, collected and weighed as 5 drams. Black observed that Chemists have often observed, in their distillations, that part of a body has vanished from their senses, notwithstanding the utmost care to retain it: and they have always found, upon further inquiry, that subtile part to be air, which having been imprisoned in the body, under a solid form, was set free, and rendered fluid and elastic by the fire.

In modern terms, 7.8 g of magnesium carbonate (MgCO3, magnesia) upon heating to high temperature yielded 3.4 g of product magnesium oxide (MgO) in comparison to an anticipated 3.7 g if (a) both starting material and product were pure, if (b) the decomposition reaction were complete, and if (c) the measurements were accurate and precise. In a separate experiment, Black dissolved a weighed amount of calcined (decomposed by high heat) magnesia in spirit of vitriol (sulfuric acid). This would produce soluble magnesium sulfate (MgSO4). He then added large amounts of a mild alkali, either sodium carbonate (Na2CO3) or potassium carbonate (K2CO3), which re-separated, by precipitation, the solid magnesia (MgCO3). The regenerated magnesia weighed nearly 100% of the original amount, showing almost complete recovery of the original material. The recovered magnesia again effervesced with acids, bubbling off “gas” just as readily as did the original material. Black concluded that the “air” given off in acids is “fixed” in the magnesia, released upon strong heating, and returned by combining with something in the alkali, the solid carbonates as we now know them. Hales had previously shown that gas could be released from the mild alkalis upon the addition of acid. Black’s careful quantitative measurements demonstrated that the “air” or “gas” is fixed in the solid magnesia and lost upon heating, justifying the label “fixed air” for the fluid substance, now known as carbon dioxide. Black’s experiments showed, for the first time, that a gas could combine with a solid, something thought impossible until then. Among the many other

20

The Legacy of Carbon Dioxide

experiments on “fixed air” performed by Black was a very simple one in which he demonstrated that a candle would not burn in it, dramatizing the idea of his label “fixed air.”

Joseph Black

As an aside, the Oxford English Dictionary has some further interesting citations from the early to mid-1700s of references to choke-damp and styth (or stythe) as present in coal mines and elsewhere. The terms were used for CO2-laden gas and continued in use up to the close of the nineteenth century.

CAVENDISH In 1766, Henry Cavendish, a British scientist born in Nice, France, published his first article with the Royal Society on “Factitious Airs,” detailing a series of experiments that referred to artificially produced substances. Among many of his discoveries, the one for which he is most known is the discovery of hydrogen gas. In later years, he also was the first to determine that oxygen was 20% of air. But for us reflecting briefly on his science now, his experiments determined the density of carbon dioxide gas. An ancillary contribution was his noteworthy observation that the gas could extinguish flames.

Discovery

21

HAMILTON Sir William Hamilton, an architect and British envoy to the court of Naples, became an expert in the actions of earthquakes and volcanoes while living in Italy. In 1770, he wrote to the Royal Society and had published in the proceedings some “Remarks upon the Nature of the Soil of Naples, and Its Neighborhood.” In referring, at first, to records of the eruption of Vesuvius in 1631, Hamilton writes The nature of the noxious vapours, called here mofete, that are usually let in motion by an eruption of the volcano, and are than manifest in the wells and subterraneous parts of its neighbourhood, seem likewise to be little understood. From some experiments very lately made, by the ingenious Dr. Nuth, on the mofete of the Grotto del Cane, it appears that all its known qualities and effects correspond with those attributed to fixed air. Just before the eruption of 1767, a vapour of this kind broke into the king’s chapel at Portici, by which a servant, opening the door of it was struck down. About the same time, as his Sicilian majesty was shooting in a paddock near the palace, a dog dropped down, as was supposed, in a fit; a boy going to take him up dropped likewise; a person present, suspecting the accident to have proceeded from a mofete, immediately dragged them both from the spot where they lay, in doing which, he was himself sensible to the vapour; the boy and the dog soon recovered.

Hamilton smartly recognizes the intimate relationship between Joseph Black’s fixed air, first described 16 years earlier – our carbon dioxide – and volcanic eruptions.

PRIESTLEY Joseph Priestley was born near Leeds, England, in 1733, trained for the ministry, and became ordained before his 30th birthday. He had no formal training in science but was self-taught. On a trip to London, he met Benjamin Franklin and was enticed into a deep interest in electricity. Priestley published his first scientific paper “The History of Electricity” in 1767. Very soon, he launched a prolific career studying gases, studies that actually began with experiments on carbon dioxide. This departure from his interest in electricity was due to his observations of the Meadow Lane Brewery neighboring his living quarters in Leeds. Priestley recognized that the brewery offered an abundant supply of fixed air and began his studies by trying to accumulate it using the gas-collecting device originally designed by Stephen Hales. The problem, though, was that fixed air was rather soluble in the water over which it was supposed to be collected. Priestley’s first publication in chemistry was on how to carbonate water, mimicking sparkling mineral waters and restorative spas. For this simple precursor to the carbonated beverage industry, the Royal Society awarded him the prestigious Copley Medal. The title page of his publication is reproduced here.

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The Legacy of Carbon Dioxide

Priestley, despite his lack of formal scientific education, was exceedingly clever. To circumvent the loss of soluble carbon dioxide while being collected over water, he devised a variant apparatus using liquid mercury in place of water. Liquid mercury does not dissolve carbon dioxide.

In 1774, Priestley heated a sample of red calx and “discovered an air five or six times as good as common air.” Red calx is mercuric oxide, HgO, one atom of mercury (element 80) combined with one atom of oxygen. Its decomposition allowed Priestley to collect the nearly pure oxygen gas which proved to be insoluble in water, enhanced the burning of a candle, and supported a mouse sealed within an airtight chamber as shown in the illustration. It is for the discovery of oxygen that Priestley is most widely known, although he didn’t recognize its role in chemistry at the time, a gap that his contemporary, the Frenchman Antoine Lavoisier, filled.

Joseph Priestley

We can proceed with carbon dioxide now that its discovery has been accommodated.

4

Structure

Everything has its beauty but not everyone sees it. CONFUCIUS

CO2 Carbon dioxide is a compound, a molecule containing more than one kind of atom. It consists of one carbon atom and two oxygen atoms. There are other compounds of carbon and oxygen that are known, some stable, some very reactive. The second most common carbon–oxygen compound is the infamous toxic gas, carbon monoxide, with one carbon and one oxygen. Some of what we know and need to know about carbon dioxide is its physical science, mostly its structure. Carbon dioxide is electrostatically neutral; that is, it is uncharged. But this does not mean that there are no regions of the molecule that possess charge, only that those positive and negative charges add up to zero. There is a property of atoms, when combined in molecules, that indicates the atom’s relative tendency to attract valence electrons to it and thereby acquire a slight negative charge, leaving a deficit of charge or buildup of positive charge. It’s called the atom’s electronegativity. The more electronegative an atom, the more the atom will pull electrons toward it, albeit only slightly. Oxygen is one of the most electronegative of all elements. There are five conceivable bonding arrangements of one carbon and two oxygens in a single molecule. Representing the chemical bond by a connecting bar and the atoms by their appropriate alphabetical symbols, the possibilities, all planar, are shown here.

One of the properties of a molecule determined by its geometry and the electronegativities of its constituent atoms is called its polarity. Actually, it’s called the electric polarity. In a sense, it is similar to the magnetic polarity of our planet Earth with its north magnetic pole and south magnetic pole. If a molecule has a north electric pole (with buildup of negative charge) separated from a south electric pole, the molecule would possess two poles, a dipole, and be said to be a polar molecule. Of course, the molecule doesn’t know north from south. All that’s needed is a separation of negative charge at one region from positive charge in another region. The greater the amount of charge and the greater their separation, the greater is the polarity of the molecule. Carbon dioxide is known from its behavior to be a non-polar molecule. It does not have a dipole. This fact alone eliminates all the possible structures for carbon dioxide except the linear O–C–O. The linear structure, because of its symmetric form, has the center of negative charge and the center of positive charge at the same location. Each of the other possible structures would be polar. There is other evidence that the linear, symmetric geometry is correct as well, and there is no contradictory evidence. Calculations, beyond our focus here, can indicate where the electrons are in the carbon dioxide molecule. Shown here in a “ball and stick display” are the partial electrostatic charges (expressed in units of the charge of a single electron) at each atom calculated from a theoretical description. 23

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The Legacy of Carbon Dioxide

Although each carbon–oxygen connection itself is polar in the sense that there is a positive pole and a negative pole, the two carbon–oxygen dipolar arrangements oppose each other. The effect cancels out, giving carbon dioxide a non-polar character. There is no center of negative charge separated from a center of positive charge. The center of negative charge is exactly midway between the oxygen atoms which is where the center of positive charge falls. Finally, using the same theoretical approach that yielded the electrostatic charges on carbon dioxide, we can construct an outline that envelops most of the valence electron distribution in the molecule. Red on both ends indicates a concentration of negative charge. The colors correspond to a gradation in charge swinging to the center blue for positive charge.

The importance of the charge locations and magnitudes on the carbon dioxide cannot be overemphasized. Essentially, all physical properties of the molecule depend in subtle ways on the charges. Since like charges repel and opposite charges attract, behavior which involves the interaction of one carbon dioxide with another carbon dioxide or with any other structure involves the relative orientations of the molecules at contact and whether or not they involve attractions or repulsions. Boiling points, pressure of the vapors, solubility, and other behaviors all derive from the picture. Conventionally, a molecule such as carbon dioxide is pictured as if the atoms were rigidly locked into place by something pictured to represent the chemical bond. While there certainly is a quintessential aspect of molecular structure called the chemical bond, it is in no way supposed to be constructed as a rigid arrangement. The bond is somewhat slack and allows the atoms to jostle around slightly, a motion referred to as vibration. Depending on how many vibrations per second, how fast they are, and on the masses of the constituent atoms, one can imagine there will be a vibrational energy associated with the molecule. You would think that if the temperature were reduced, the molecule could give up vibrational energy. This is certainly so. But unlike the everyday concept that if we cool the molecule enough, the motion should stop completely, well-established theory tells us that such motion cannot stop. There will always be a residual vibrational motion even at the absolute zero of the temperature scale, −479°F or −273°C. The energy associated with the residual motion is called the zero-point energy and is a direct consequence of an uncertainty principle (or indeterminacy principle) recognized first by the German theoretical physicist Werner Heisenberg during the mid-1920s.

Structure

25

Vibrational motions in carbon dioxide can only increase in nearly equal jumps of energy, like bounding up a staircase, each step representing a higher and higher vibration frequency. The energy can be provided by light – electromagnetic radiation – that occurs in the infrared region of the spectrum. Infrared light has slightly less energy than that linked with visible light (and much less than that of ultraviolet [UV] light). If we bathe carbon dioxide with infrared radiation of different frequencies, the maximum effect occurs at a wavelength of ~15 micrometers, corresponding to a frequency (of light’s oscillating field) of about 20 trillion cycles per second (20,000 gigahertz). A microwave cooking range operates at about 2.5 billion cycles per second. There is an additional key requirement here. For the oscillating electric field of light to be effective in producing vibrational jumps there must be an oscillating dipole in the absorbing molecule. This is a requirement for the process we’re discussing, although we won’t attempt to explain its origin. But carbon dioxide has no permanent dipole as we pointed out already. However, Heisenberg’s uncertainty principle guarantees that carbon dioxide must vibrate with at least its zero-point vibrational energy in each of its several modes of motion. It is therefore guaranteed to have an oscillating dipole in bending and asymmetrical stretching vibrations. Even though the average dipole is zero, there is a flip-flopping dipole.

The absorption of light corresponds to an energy match that would allow a jump in the vibrations from 10 to 30 trillion cycles per second, a wavelength of 15 micrometers corresponding to the energy* needed to accomplish this. Because the remaining vibrational mode, the symmetrical stretch, does not involve an oscillating dipole, the molecule will not absorb the infrared light that would otherwise perfectly match the energy needed to boost the vibrational frequency for this mode. Incidentally, this is the precise reason that molecular nitrogen (N2) and molecular oxygen (O2) do not absorb infrared radiation. Their vibrations, by the very symmetry of the two molecules, do not have oscillating dipoles associated with them. They are transparent to infrared light. As esoteric as this recent discussion may seem, it is actually the embedded Heisenberg uncertainty principle that explains why carbon dioxide is so intimately associated with being a greenhouse gas, effectively absorbing infrared radiation emitted outward from Earth’s surface rather than it passing directly into space. More on that later.

OTHER PROPERTIES OF CARBON DIOXIDE Carbon dioxide is one-and-a-half times as heavy as air. Since CO2 doesn’t support combustion and is easy to produce, it is used in many fire extinguishers as a way of suffocating a flame, starving the flame of oxygen. The gas may be liquefied by cooling to −18°C at a high pressure of about 20 atmospheres or, alternatively, at “room temperature” 21°C and a still higher pressure of about 67 atmospheres. At temperatures greater than 31°C and pressures greater than 74 atmospheres, carbon dioxide will have been compressed to the point where liquid and gas are indistinguishable and one has * 15 micrometers wavelength is equivalent to 1.3 × 10 −20 joules or 3,200 billionth of a billionth calories for one molecule or 44 calories per gram.

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The Legacy of Carbon Dioxide

what’s just called a fluid. The conditions under which this first occurs are referred to as a critical point and so the fluid phase is called supercritical carbon dioxide. Commercial uses of supercritical carbon dioxide are growing extremely rapidly. The fluid is recyclable and easily purified. It has replaced ultrapure water in computer chip manufacturing and is used to clean electronic parts and mechanical parts. Chlorinated organic solvents such as trichloroethylene (TCE, also known as trichlor) used in dry cleaning are toxic, but are being replaced with supercritical CO2. The fluid is also used as a solvent in a variety of specialty situations, the most well-known of which is the removal of caffeine from coffee.

OTHER CARBON–OXYGEN MOLECULES Among other carbon–oxygen compounds, there is, of course, carbon monoxide (CO). Less well known are small clusters of these molecules. Combinations of two and three carbon monoxides and carbon dioxides form molecules whose formulas could be written as C2O2, C3O3, C2O4, and C3O6. A carbon suboxide has been known for years and has the formula C3O2. Measurements of light passing through interstellar molecular clouds have been used to identify dicarbon oxide (ketene) and tricarbon oxide, C2O and C3O, respectively. Under normal conditions, in the atmosphere or everyday environment, these would react and disappear. Finally, the tetra-hydroxide of carbon exists in theory. It would be the product of reacting carbon dioxide with water:

CO2 + 2H 2O → C ( OH ) 4

And if favored as a reaction, the tetra-hydroxide would completely change the nature of the world. Carbon dioxide and water are discussed in Chapter 10. Further reaction of this hypothetical substance with basic elements leads to the formation of orthocarbonates, a known example of which would be sodium orthocarbonate, Na4CO4.

PHASES Elemental carbon has two very well-known phases, assemblies of uniform properties, distinct and separable from others: graphite, the writing substance in a so-called lead pencil, and diamond, which is formed in certain geological environments deep beneath Earth’s surface under very high pressure. A third form has achieved notoriety recently and is associated with the 1996 Nobel Prize in Chemistry. The form is buckminsterfullerene, or “bucky ball.” Carbon dioxide is in the gas phase…under ordinary circumstances. However, if you drop the temperature to −78.5°C (−109.3°F), carbon dioxide freezes into a solid phase. That solid phase is commonly referred to as dry ice. Warm the dry ice, and it converts back into the gas. Unlike most other solids, it does not melt into a liquid first. (The conversion of a solid into a vapor without melting first into a liquid is referred to as sublimation.) The liquid phase of carbon dioxide does exist, but only at pressures above five atmospheres. And at that pressure, the temperature must be no lower than −56.6°C. At even higher pressures, solid carbon dioxide can take on different forms. One form is known as carbonia and was discovered recently, in 2006. It is analogous to the glass form of silicon dioxide. Multiplicity of solid structures is a fairly common phenomenon. Ice, to date, has nine different solid forms (one of which was the basis of Kurt Vonnegut’s novel Cat’s Cradle).

Structure

27

In this diagram of temperature versus pressure with the various solid phases indicated by Roman numerals, the solid lines indicate where two phases would co-exist, like ice and water at 0°C. Note that if carbon dioxide were just above 100,000 atmospheres and 1,000 K temperature (the diamond on the figure), it would be CO2 (solid) phase IV. If the pressure were reduced, moving horizontally to the left on the diagram, the solid would become liquid CO2. That is, it would melt without the application of heat. Most material, water ice being an exception, behaves like carbon dioxide in this way. But since water ice is the substance we have the most experience with, the aforementioned melting phenomenon seems counter-intuitive to our experience. Carbon dioxide may take several other distinct solid forms. Studies* on these have recently advanced our knowledge but are still not completely resolved. Among observations are two that are particularly interesting. At high pressure, carbon dioxide may convert to a “superhard” solid structure. Additionally, there is the possibility that solid carbon dioxide may exist in Earth’s mantle as a structure similar to its analog, quartz: SiO2. The more CO2 is studied, the more surprises are revealed.

* J. Sun et al., “High-pressure polymeric phases of carbon dioxide,” Proc. Natl. Acad. Sci. 106, 6077 (2009).

5

Radiocarbon and Its Dioxide

Carbon-14, the long-lived carbon isotope, is the most important single tool made available by tracer methodology, because carbon occupies the central position in the chemistry of biological systems. MARTIN KAMEN Co-discoverer of carbon-14 Longest lived among carbon’s radioactive isotopes is carbon-14. This is a naturally occurring radioactive substance and, over the years since its discovery by Americans Martin Kamen and Sam Ruben in 1940, has served as a powerful analysis tool in a variety of applications. Those applications are part of the carbon dioxide story and we will explore some of these after we look at why there is any carbon-14 around when its average lifetime is about 8,000 years yet our planet is over 4 billion years old.

COSMIC RAYS The process of removing one or more electrons from an atom or molecule is called ionization. (Adding an electron to form a negatively charged ion is not referred to in this manner.) High energy particles emitted by the sun reach Earth’s atmosphere and there cause ionization of atoms and molecules. Many more particles hitting Earth come from outside the solar system, that is, are galactic or extragalactic. They are mostly protons – 90% – with another 9% being alpha particles, and the rest are yet heavier nuclei. (Their energies are so great that their detailed behavior needs to be understood using Einstein’s theories of relativity.) In the atmosphere, via nuclear reactions, they can produce showers of other nuclear particles including neutrons. Cosmic rays are ultimately responsible for a variety of phenomena among which are the auroras seen in the more polar latitudes of both northern and southern hemispheres. They influence not only the chemistry of the atmosphere, transformations from one molecular arrangement to another, but some nuclear transformations as well. As far as our book’s topic is concerned, cosmic rays provide a fortuitous by-product with many benefits.

PRODUCTION OF RADIOCARBON The most common atom in the atmosphere is nitrogen, element number 7. Nitrogen exists mostly as a gas whose constituents are two nitrogen atoms chemically bound together: molecular nitrogen, N2. All but about 0.4% of naturally occurring nitrogen atoms have mass number 14. The mass number implies that in addition to the seven protons in the nucleus, there are also seven neutrons for a total of 14 elementary particles. A rare collision in the atmosphere of a fast neutron from cosmic rays with the nitrogen-14 nucleus sometimes happens in such a way that a proton is knocked out of the nucleus and the neutron remains. You could picture this akin to what happens when a fast billiard ball strikes head-on one that is stationary. The struck one recoils quickly in the original direction and the incoming ball remains behind in its stead. This simplified picture means that the struck nucleus with its components of seven protons and seven neutrons is converted into a product with six protons and eight neutrons. That final combination is the isotope carbon-14.

29

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The Legacy of Carbon Dioxide

Even more often, a slow neutron is captured to form 15N, depositing enough energy that a proton is subsequently ejected a very short time later to produce the 14C. Carbon-14, sometimes called just radiocarbon, is practically identical in its chemical behavior to carbon-12 and carbon-13 but is unstable with respect to radioactive decay at a very slow rate. A carbon-14 lives an average of over 8,000 years before being transformed by a spontaneous nuclear transmutation. That is by far long enough for the carbon-14 to become chemically incorporated into atmospheric carbon dioxide, for example, and to have its share of the myriad of chemical processes that occur as part of the so-called carbon cycle and to serve as a tracer or proxy of those processes. It also means that the vegetation we consume unavoidably contains radiocarbon (unless the plants are tens of thousands of years old). The average human weighing 70 kg is about 12.6 kg carbon. The trace amount of radiocarbon there would give an average of 3100 decays per second, non-stop, and constant. In radioactivity units, we are talking about 85 nanocuries (or 3100 becquerels). Production and decay of carbon-14 are more or less balanced in the atmosphere at the present. The average production rate is 2.2 atoms per second for every square centimeter, 90% from solar cosmic rays, the remainder mainly galactic. An equilibrium amount of 15 decays per minute for every gram of carbon in the atmosphere is obtained. The atmosphere contains a steady total amount of about 6 megacuries of radiocarbon – naturally.

ISOTOPE EFFECTS In the “Production of Radiocarbon” section, there was a subtle hedge in the comment on the chemical behavior of the different carbon isotopes, saying they were “practically identical.” There are, in fact, very slight but definitely measurable and understandable differences in the chemistry of different isotopes of the same element due to their slightly different masses. A more thorough explanation of the origin of this fractionation effect is deferred to Chapter 20, but presenting some observations on the effects of different isotope masses make sense now. In working with carbon-12 (the 99% abundant, stable isotope) and switching to carbon-13 (also stable) properties will alter very slightly. The 13C/12C abundance comparison is expressed relative to a standard sample in terms of percent difference of that ratio from that reference standard ratio. (Professional researchers who study this topic, instead of percent deviation, use per mil: ‰ = per mil or parts per thousand.) Atmospheric carbon dioxide has a fractionation of −0.9%. If the standard had exactly 1% 13C (ignoring the fact that it is not precisely 1%), a −0.9% fractionation means that the air sample instead has* 0.991% 13C. The “−0.9%” deviation indicates that there is relatively less 13C than in the standard. Carbon in desert cactus, to pick an example, has a fractionation of +1.7%. Leaves from trees have typical fractionations of +2.7%. The three examples quoted actually have a range of measured values. There are also many more examples of representative fractionations for carbon (and oxygen). Not unexpectedly, 14C will have an isotope fractionation effect that is even greater than that for 13C (since its mass differs from that of 12C by more than does that of 13C).

* The standard ratio is 1%/99%. The sample ratio gives a fractionation difference (x – 1/99)/(1/99) = –0.9 in % or x = 0.01001 the 13C-to-12C ratio in the sample. Since those mass 12 and 13 isotopes add to 100%, the 13C becomes 0.991% of carbon.

Radiocarbon and Its Dioxide

31

RADIOACTIVE DECAY OF CARBON-14 Carbon-14 is unstable. It is heavier – more massive – than nitrogen-14. Each of these isotopes has a mass number of 14, representing the total number of protons and neutrons in the nucleus. But the atoms’ actual masses differ by about one-thousandth of a percent. Carbon-14 can change to stable lower mass* nitrogen-14 because a pathway is open to allow that change. The pathway is radioactive beta-decay in which the six protons and eight neutrons in the nucleus spontaneously change into seven protons and seven neutrons. Effectively, one neutron has converted into a proton, emitting a negatively charged electron from the nucleus. That electron gets a special name, a beta particle.† Beta particles can be detected with sufficient sensitivity that corresponds to 10,000 year-aged samples. (Rather than exploiting radiation measurements, 14C atoms themselves can be measured by accelerator mass spectrometry, improving the sensitivity enough to samples of age 50,000 years.) The beta particle is identical in all respects except history to an electron. For radiocarbon, the probability of this beta decay, although guaranteed, is fairly low. The rate is a property of the nucleus and corresponds to about a 1.2% chance of decay in a century for any carbon-14 atom. It is instructive here to follow what happens if we start with a large number of such nuclei. Suppose we initially have 100% 14C. After one century, 1.2% of these will have been lost, becoming product 14N and leaving behind the remaining 98.8% 14C yet unchanged. Over the course of the next century, 1.2% of the latter decay into 14N, leaving behind 97.6% of the original 14C. That is, if 1.2% decay, 98.8% survive the century. We started with 98.8% of the original and if 98.8% of these survive into the second century of elapsed time, that’s where the 97.6% figure comes from. The next century would leave 96.4% of the original; the next would leave 95.3% and so on. After one millennium, 88.6% of the original 14C would remain; after two millennia 78.4% would be left. If we graph this behavior, the figure shown results.

This type of behavior follows what is called an exponential curve. What you might notice is that the amount of time it takes for half of the 14C to decay is between 5,000 and 6,000 years. The actual figure is 5,700 years and this time is referred to as the half-life of 14C. No matter when you begin a measurement, in 5,700 years half of the 14C will have turned into 14N. Carbon-14’s average life is around 8,200 years, a value that is unaffected by anything in the surroundings. After ten lifetimes (82,000 years), 99.98% of the original amount has decayed.‡ A “lifetime” does not imply that the substance is extinguished as if it were living. Rather, the meaning is that this is the average time a nucleus remains unchanged. Some decay earlier, some later…randomly. * Lower mass = lower energy, vis-à-vis E = mc2. † An additional particle, an antineutrino, is also emitted but that detail is not germane to our discussion. ‡ A legitimate question is the impact of the nitrogen-14 produced. There is so much more nitrogen in the atmosphere that the contribution from radiocarbon decay has negligible effect. Six megacuries of carbon-14 produce (6 × 106) (3.7 × 1010) nitrogens per second or fewer than 2 g per year. Even millions of years would have minimal effect.

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The Legacy of Carbon Dioxide

Just to try to illustrate that the half-life of an unstable isotope is a characteristic of that particular isotope, a few other half-lives are worth mentioning. Uranium-238 has a 4.5-billion-year halflife. Naturally occurring (even in “substitute salt,” potassium chloride, KCl) potassium-40 has a 1.3-billion-year half-life. Radon-222, the naturally occurring isotope of the radon of health concern in basement gas seepage, has a 3.8 day half-life. Fluorine-18, used in PET scans, has a 110 minute half-life. Recently discovered and named superheavy element (number 118), oganesson has a 294Og half-life of 0.0007 seconds. Because the isotope disappears at a certain rate, the 14C at first accumulates. But then an equilibrium is reached in which the rate of accumulation is balanced by the rate of disappearance. The amount of 14C in the atmosphere would then remain constant. The balance between appearance and disappearance is called a “steady state” situation. Carbon-14, because of its nearly perfect imitation of natural 12C chemistry, can be used to determine age, a fact that was first recognized by American chemist Willard Libby. The technique had proven so valuable to archaeology and paleontology, the study of former life forms, usually via fossils, that Libby was awarded the 1960 Nobel Prize in Chemistry for his discovery. Let us look at a tree as an illustration of the radiocarbon dating method. While the tree is living, carbon dioxide from the atmosphere is used to produce wood, basically the substances cellulose and lignin, containing lots of carbon. Not only does that carbon consist mostly of 12C with about 1% 13C, but also of some tiny amount of 14C that was present in the air, the 14C having been produced by cosmic rays. To a good first approximation, the amount of 14C present in the air has been constant over many millennia. If so, then the ratio of 14C to 12C will be nearly constant in living parts of the tree, older parts having very slightly less 14C than younger parts of the same plant.* In wood where carbon is no longer being incorporated, the 14C/12C ratio begins to drop because of radioactive decay. When the tree dies, the 14C/12C ratio drops everywhere, always at the rate determined by the 14C half-life. A measurement then of the remaining 14C through its decay rate directly tells you how much 14C you have in a sample. If you isolate 1 g of pure carbon from newly formed wood, you obtain (approximately) the equilibrium 15 decays per minute. If you have another piece of wood and you do the same experiment, but the decay rate of 14C is measured to be 7.5 decays per minute† in a gram of pure carbon, this is an indication that the 14C has not been replenished for an amount of time equal to one half-life: half has decayed and half remains as seen in the graph. The sample is 5,700 years old. That is how long it has been since growth ceased. The age of that piece of wood is 5,700 years.

AGE CALIBRATIONS That simple illustration of radiocarbon dating began with the proviso that atmospheric radiocarbon abundance is and was always the same. Cosmic ray intensity bringing about radiocarbon production has not been absolutely constant though. And we’re not talking particularly about the fluctuations associated with the 11-year sunspot cycle which amounts to about 0.1%. More significant fluctuations, including alterations in Earth’s magnetic field, which can deflect cosmic rays, force corrections onto the interpretation of the radiocarbon-determined ages. Indeed, it was the Dutch physicist Hessel de Vries who in 1958 was the first to demonstrate that radiocarbon ages were imperfect. Fortunately, for the most recent seven or so millennia, such corrections are confidently determined through a calibration using “dendrochronology,” a technique invented by the American astronomer Andrew E. Douglass just before the start of the twentieth century. Douglass discovered the apparent correlation between climate and plant growth from evidence in tree rings. Experience and common sense tell us that thick rings correspond to productive growing seasons. History

* From earlier, a century difference in age corresponds to a 1.2% difference in 14C content. † There cannot be a fractional decay, but 7.5 decays per minute means the same as 15 decays in 2 minutes, for example.

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of interest in tree rings* extends back as far as Aristotle’s student and successor, Theophrastus (370–285 B.C.E.), considered by some to be the “father of botany.” Leonardo da Vinci recognized the relationship between tree rings, moisture availability, and past weather. In the mid-1700s, Carolus Linnaeus (“father of modern taxonomy”) noted that tree rings record historical weather patterns and called rings “chronicles of winters.” Charles Babbage (“father of the computer”) in 1837 suggested that overlapping patterns of tree rings could enable construction of ancient chronologies.

Andrew Douglass demonstrated that there were matching “records” in tree rings from collections of trees. He used overlapping growth patterns as a dating technique. A schematic representation of the technique is diagrammed here.

Over the years, this idea has been extended to include radiocarbon data from layered lake sediments called varves. Their similarity to tree rings was noted by Dutch–Swedish geologist Gerard de Geer in the late nineteenth century. Establishing the chronology of the layers, though, is complicated by the occasional double layers in one year or missing layers due to the vagaries of seasonal growth. * R. Wimmer, “Arthur Freiherr von Seckendorff-Gudent and the early history of tree-ring crossdating,” Dendrochronologia 19, 153 (2001).

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In a similar vein, studies of coral, which also have growth rings (illustrated here), have proven useful, as have radiocarbon measurements of stalagmites in limestone caves. In the latter case, radiocarbon readings are shifted from the ages of the stalagmite deposits because of the time expended to produce the required carbonate from overlying organic-rich soil. Furthermore, the porous nature of the stalagmites allows “young” carbonate to seep into “old” accumulations and blur the age information.

The various data points in the figure show the calibrated “radiocarbon” age along the vertical axis, most often slightly underestimating the true age. There are slightly different calibrations for the two hemispheres. Time resolution is five years for just over the first 10,000 years. Radiocarbon age is what is extracted from the portion of carbon that is 14C remaining in the sample, calculating the time needed to drop from the current value to that measured one. The horizontal axis is the actual age of a sample – determined by other, independent measurements or records – whose radiocarbon content will be measured. For the first 10,000 years, the age comes from dendrochronology. Subsequently, ages come from other radioactive isotope dating techniques that do not depend on constant cosmic ray intensities over time and also from measuring sediment layers in lakes and growth rings in corals. The straight line shows where measurements of carbon-14 would be expected to directly yield accurate ages if no corrections proved necessary. Prior to 10,000 years ago, the general interpretation is that cosmic ray intensity was less than more current levels, producing less 14C and giving younger ages. A near shutdown of Earth’s magnetic field several tens of thousands of years ago is hypothesized as an explanation. The proposal is supported by magnetic measurements in deep sea sediments and by 10Be measurements (another radioactive species generated by cosmic rays). Naturally, the amount of nitrogen in the atmosphere from which the radiocarbon is generated is understood not to have fluctuated over these millennia.

Radiocarbon and Its Dioxide

35

DEVIATES Fluctuations in cosmic ray intensity, and thereby the production of radiocarbon, can occasionally have confounding effects. For example, in the calendar age versus radiocarbon age calibration graph, the 14C “clock” is seen to have stalled for several thousand years around 26,000–31,000 years ago (the curve flattens) possibly and probably due to below average radiocarbon production. Prior to that, starting around 31,000 years ago, the “clock” accelerated, presumably reflecting above average production of radiocarbon. Standards are needed for precise and accurate (true) age determinations. By combining the 14C measurements and sequencing of tree rings through successive seasons in trees whose lives overlapped at some identifiable point or points as shown in the earlier schematic diagram, calibration can be accomplished. The technique enables determination of a calibration extended over many millennia. The age of a tree wood ring is determined by counting rings backward from the present (or from a known date), then matched with the age determined just from 14C content of the ring to establish a calibration or correction. An example of calibration that covers just the last several hundred years in very precise detail is shown here.

On the horizontal axis is the true date of the sample being analyzed for radiocarbon in tree rings determined by dendrochronology (counting and assigning rings). On the vertical axis is the age calculated from measured 14C content and the 14C-to-total-carbon decay factor. The straight line through the intersection of the two axes indicates where the data would have been if there were no corrections necessary, that is, if the radiocarbon date exactly mirrored the true date. You see that corrections are necessary. For example, the tree ring that grew during the year 1600, when analyzed for radiocarbon to determine its age, gave (on the horizontal) 1600 also. But for the tree ring that actually grew in 1700 according to the tree ring counting result on the horizontal axis, an apparent age from radiocarbon dating is 1850 (for the vertical axis) implying that too much radiocarbon is present (150 years’ worth) compared to the correct age. For tree rings that date at 1850, this means there is 1.8% too much carbon-14 in the uncorrected radiocarbon description.*

OTHER COMPLICATIONS If an undetermined sample of wood, say, has its 14C/C level measured and yields a radiocarbon age of 150 years (prior to the reference year, 1950) or a date of 1800 by extrapolating backward using the known half-life, the calibration data show that the true age in this case is ambiguous. The measured 14C/C level on the calibration would agree with dates of 1675, 1725, 1820, and 1925 equally well. These correspond to ages both older and younger than the radiocarbon level indicates. * Recall from earlier that a century corresponds to a 1.2% carbon-14 change.

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What is displayed next is the standard correction for radiocarbon in tree rings determined by dendrochronology and standard tree samples from the previous illustration.* The peak at 1.8% for the correction factor corresponds to the year 1700 as just discussed. Note, too, that at 1600 the correction happens to be 0%.

An even more detailed study of deviations in atmospheric radiocarbon production was initiated by a 2012 study of tree rings in cedar by a team of scientists in Nagoya, Japan.† The scientists found a sudden jump in radiocarbon associated with the year 775 CE using tree ring dendrochronology with one to two year time resolution, a considerable improvement in what is used in the official calibration standards that tie radiocarbon ages to actual ages. Questions about the cedar wood origin and date naturally arose and answering those questions proved to provide additional valuable insight. Suggestions about the cause included a strong solar burst of cosmic rays; a supernova gamma ray burst; the impact of a comet on the sun producing a coronal shock; and a comet whose high nitrogen content would have been a source of radiocarbon via nuclear reactions. The following year, identical dating and amplitude effects were found in German oaks thousands of miles from Japan in studies by two independent laboratories.‡ Also identified in the samples were jumps in 10Be and 36Cl radioisotope abundances, proxies for cosmic rays. The timing of the phenomenon matches a cluster of aurora borealis reports in Chinese chronicles. Calculations implied that, if a comet impact were responsible, the object would have to have been greater than a kilometer in size and the effect would have been basically confined to one hemisphere of the planet. A 2014 international collaboration§ performed yet another series of measurements, this time on larch trees from northern Siberia and bristlecone pine from California. Annual resolution in tree ring counting matches the 1.5% radiocarbon jump accurately and precisely as shown in the figure for the five global investigations.

* In actuality, isotopic fractionation of 14C is also included in the values displayed. This is just an indication of the very slight differences in how plants distinguish between the two isotopes of carbon being determined, 14C and 12C, namely, there is not a perfect correspondence in the incorporation of CO2 between both isotopes. The fractionation adjustments are known to be accurate. † F. Miyake et al., “A signature of cosmic ray increase in AD 774–775 from tree rings in Japan,” Nature 486, 240 (2012). ‡ G. Usoskin et al., “The AD775 cosmic event revisited: the Sun is to blame,” Astronomy and Astrophysics 552, L3 (2013). § A. J. T. Jull et al., “Excursions in the 14C record at A.D. 774–775 in tree rings from Russia and America,” Geophys. Res. Lett. 41, 3004 (2014).

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The same international collaboration found identical radiocarbon spikes in Kauri wood from New Zealand and also in corals, dated by counting growth rings, in the South China Sea. Some questions were raised about missing tree rings and erroneous date assignments if growing seasons were slow enough to make a ring hypothetically indistinguishable from neighboring years’ rings. But yet another study, this one from pine trees in the Austrian Alps,* seems to obviate such concerns. The samples were from elevations of more than 2 km where low temperatures should have produced frequent slow growth seasons. Nevertheless, the dendrochronology of the observed radiocarbon spike again matched the year 775 results observed globally. All of these investigations of the phenomenon that occurred well over a millennium ago serve to lend strong confidence to radiocarbon chronology studies. The guilty solar event is arguably the strongest such event in over 11,000 years. Around the year 775, there would have been an equilibrium amount of 815 kg of 14C in the atmosphere with a normal production rate of about 6.7 kg per year. The spike seen implies a two-fold increase in production at that time (diluted to a 1.5% effect because of the much higher amount already present).

ANTHROPOCENE Since about 1900, the 14C/C ratio has been significantly disrupted by two man-made interferences. The industrial revolution started in the late 1800s. By 1900, very large quantities of fossil fuels, those that had been buried since antiquity – coal, natural gas, and oil – were burned. Since coal is many millions of years old, any 14C originally present would have decayed completely (as is confirmed by measuring various coal sources). Coal burning would therefore introduce stable C isotopes into the atmosphere beyond normal (natural) sources, diluting the 14C/C ratio from its ideally constant value in the air. This effect was recognized by the Austrian chemist Hans Suess and is named after him. Between 1850 and 1950 it is well documented that combustion of carboncontaining materials added 10% excess – radioactively “dead” – carbon dioxide to the atmosphere above and beyond what had been historically occurring. This should have been reflected as a 10% drop in the 14C/12C ratio. Yet interestingly, the radiocarbon studies of tree rings illustrated previously for the years 1500–2000 showed that the reduction was only 2 to 3%, evident in the dip of the data near the right edge of the graph. The most rational explanation for the significant discrepancy is that * U. Büntgen et al., “Extraterrestrial confirmation of tree-ring dating,” Nat. Clim. Change 4, 404 (2014).

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there must be a very effective carbon dioxide repository or “sink” sequestering the excess carbon dioxide production, a sink that is larger in its effect than is the atmosphere (ruling out causes by living species). Soil humus is a large reservoir of carbon, but the rate of exchange is too slow to account for the observed change in step with the industrial revolution. The large and very influential carbon dioxide sink is the ocean, as will be explored in subsequent chapters. The other human intervention in the radiocarbon ratio came about during the period beginning in the 1950s when atmospheric testing of nuclear weapons was, well, prolific. Most nuclear detonations, but not all, were conducted in the northern hemisphere by the United States and by the Soviet Union. These atmospheric tests produced copious bursts of neutrons which would increase the amount of 14C present over and above that due to production by cosmic rays. In the graph we show tree ring determinations* of 14C from Austria, representing the northern hemisphere in which most of the testing was done, from New Zealand, as proxy for the southern hemisphere, and from Ethiopia. Note immediately that the vertical scale, representing departures from unperturbed radiocarbon content expectations, approaches 100%. The method employed collection of carbon dioxide followed by radioactivity assay.

The phenomenon has been taken advantage of as a means of studying stratosphere–troposphere exchange, variable ocean circulation, and sea–air exchange of carbon dioxide. The difference between the two global hemispheres is not as vivid as would be expected from the northern hemisphere sites of the bomb-produced 14C. That is an indication of the extent of mixing of air with its CO2 between the two hemispheres. Testing by various nations began with fission bombs in the 1950s and in the 1960s continued with the introduction of thermonuclear (hydrogen fission–fusion–fission) bombs producing even more marked spikes. The data are zeroed from the 1940s levels as an indication of where the 14C “correction” would track in the absence of these recent man-made perturbations. By looking at these curves, you are able to discern that it took very roughly 15 years * K. Dutta, “Sun, ocean, nuclear bombs, and fossil fuels: Radiocarbon variations and implications for high-resolution dating,” Annu. Rev. Earth Planet. Sci. 44, 239–75 (2016).

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39

for the 14C peak to fall to half its value. In comparison to the 14C half-life of 5,700 years, the shorter disappearance time is a clear indication of the existence of physical and biological mechanisms for removing atmospheric carbon dioxide. It does not indicate that half of the carbon dioxide is removed in 15 years, but that half is turned over, cycled to an extent with fresh carbon dioxide, at that rate. One of those carbon dioxide sinks – no surprise here, as we mentioned – is the ocean. Radiocarbon measurements of the oceans themselves have been made. Displayed in the accompanying figure is 14C from calcium carbonates* in coral reefs that can similarly be dated by counting age rings in the coral. Coral grows only at relatively shallow marine depths. The radiocarbon spike from these coral near the surface proves to serve as a tracer of carbon dioxide in the oceans and of the oceans themselves. In the figure, previously described corrections derived from tree rings are indicated by the broad gray slightly wavy line up to about 1950. The 5% difference from coral carbonate radiocarbon levels has to do mostly with different isotopic fractionation in coral versus that in trees. That is, typical isotope fractionation for coral, distinguishing 14C from 12C, averages −5%. What is shown by the data points is the deviation of radiocarbon content from what is ideally expected for the age of the coral analyzed. In the 1950s and 1960s (in the figure’s bracketed region), nuclear testing added 14C to the atmosphere peaking in 1963, but the effect doesn’t peak in coral until years later, in the 1980s. Note similarities and differences with the previous graphs from tree ring studies. A bit of thought lets us realize that the delayed peaking is just a reflection of the delay due to the non-instantaneous rate of exchange (recall the 15 year “half-life” for atmospheric weapon-produced 14CO2) of atmospheric radiocarbon with the oceans and the time needed for the slow growth of coral. The negative correction for older coral is also noteworthy. Radiocarbon levels of surface waters in the oceans appear 400 years older than the overlying atmosphere (that feeds trees) due to mixing – dilution – with deeper, older waters and can serve to quantify the rate of such mixing.

MODERN TWEAKS Recent atmospheric measurements of the amount of radiocarbon in atmospheric CO2 were slightly lower in the northern hemisphere compared to the southern hemisphere. This is reasonably ascribed * Carbonates, discussed more thoroughly in Chapter 13, are combinations of carbon dioxide with metal oxides.

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to differences in non-radioactive carbon dioxide dilution from the burning of fossil fuels in the north versus the south. Nevertheless, part of the effect is tempered by the release of extra radiocarbon from nuclear power reactors, prevalent in the northern hemisphere. The observation is a turnaround from preindustrial levels where tree ring measurements implied less radiocarbon in the southern hemisphere. This latter result, arguably normal, arises from the upwelling of deep marine carbon dioxide, depleted in radiocarbon by decay over long residing time, and subsequently released back to the southern hemisphere air to dilute its carbon-14 concentration. An intriguing observation reported at the 21st International Radiocarbon Conference held in Paris in 2012* was made of air sampling in Indianapolis, United States. A sharp downward spike in local radiocarbon was spotted. The investigators realized this was due to the famous Indianapolis 500 auto race which had just occurred. But it was not emission from race cars that produced the spike, but rather the 400,000 spectators who drove there and back consuming fossil gasoline and diluting the local 14C readings. In natural science the principles of truth ought to be confirmed by observation. CAROLUS LINNAEUS

* M. Balter, “Neandertal champion defends the reputation of our closest cousins,” Science 337, 400 (2012).

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Our most basic common link is that we all inhabit this small planet. We all breathe the same air. JOHN F. KENNEDY

AIR PRESSURE You can’t really feel it, but the weight of all the air on top of you pushing down from above is what we commonly refer to as air pressure. Typical pressure of the atmosphere at sea level is a bit less than 15 pounds over a square inch or about a kilogram per square centimeter. This value varies with altitude since, obviously, the higher you go, the less air is above weighing down upon you. In the other direction, a whale at depth 1 km under water is subject to a pressure about a hundred times greater than atmospheric pressure. Since the atmosphere is pushing down with 1 kg of weight on each square centimeter of Earth’s surface, we can figure out the total weight or mass of the atmosphere. Earth’s diameter is some 8,000 miles (13,000 km). The area of a sphere whose diameter is 8,000 miles is 200 million square miles or 5.1 × 1014 m2 (or 5.1 × 1018 cm2). If there is a kilogram of air pushing down on each square centimeter, the total mass of the atmosphere is then 5.1 × 1018 kg. That there are mountains and valleys with corresponding decreasing or increasing air pressure, respectively, complicates the calculation. But ignoring their effect doesn’t change the final value enough to concern us.

AIR COMPOSITION Jean Baptiste Boussingault was a nineteenth century French agricultural and analytical chemist. Born in Paris, he spent time in Colombia, South America, and served with Simon Bolivar during their revolutionary times in the 1820s. While there, he sampled gas emissions from fumaroles near one of the many volcanoes and found carbon dioxide to be present. On returning to France, he made careful measurements of the atmosphere and was the first to determine that carbon dioxide in air was between 280 and 310 parts per million by weight. He did this in the 1830s.

Jean Baptiste Boussingault 41

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Air, we all know, is composed mostly of nitrogen. Oxygen comprises only about one-fifth of air. Argon is about 1%. In terms of how many gas molecules there are in an arbitrary volume, that turns out to be just less than 30 billion billion molecules in a cubic centimeter at sea level at 20°C. The value is both temperature dependent and altitude (pressure) dependent. Water vapor can reach as high as 4%. By the time we reach airline cruising altitudes of 10 km (32,000 ft), the number of molecules in that cubic centimeter has dropped to one-third of its sea level value. At 10 times this altitude, the density has dropped precipitously. There are fewer than a millionth as many gas molecules in the cubic centimeter at 100 km altitude as there are at sea level. Furthermore, the composition has also changed. About 3% (30,000 ppm) of the air at 100 km height now consists of atomic oxygen (O) in addition to the molecular oxygen (O2) that we are familiar with. Carbon dioxide at this altitude is roughly 250 ppm. Atomic oxygen is chemically very reactive. Doubling our altitude again, bringing us to 200 km, the most abundant gas in the atmosphere is atomic oxygen, comprising nearly half of all the species present. Nitrogen makes up most of the rest, while life-supporting molecular oxygen is present at only 3%. For reference, the space station is at about 340 km altitude. Atmospheric gases constitute quite an olio. The following table shows the global mean composition for 2011 (except for nitrogen, oxygen, argon, water, and a few others) as reported* in the IPCC5 (International Panel on Climate Change, Chapter 2 on “Observations: Atmosphere and Surface”) with its emphasis on greenhouse gases. The ± precision with which the concentrations are known is excellent.

Species CO2 CH4 N2O

ppm (parts/million) 390.48 ± 0.28 1.803 ± 0.005 0.3240 ± 0.0001

Species

ppb (parts/billion)

SF6

7.26 ± 0.02

CF4 C2F6 CHF2CF3 CH2FCF3 CH3CF3 CH3CHF2 CHF3 CCl3F CCl2F2 CClF2CCl2F CHClF2

79.0 ± 0.1 4.16 ± 0.02 9.58 ± 0.04 62.4 ± 0.3 12.04 ± 0.07 6.4 ± 0.1 24.0 ± 0.3 236.9 ± 0.1 529.5 ± 0.2 74.29 ± 0.06 213.4 ± 0.8

Carbon dioxide presently constitutes 0.04% of the atmosphere. That value can also be expressed as 400 parts per million (ppm). There are local variations, indubitably. Natural sources, especially volcanoes, emit about 0.03 atmospheres of carbon dioxide per million years (My), as will be discussed in Chapter 13. To achieve equilibrium if volcanic activity were the only significant source, a steady level of carbon dioxide, 0.03 atm/My, would also have to correspond to the disappearance rate so that there is no net gain or loss (on average). Inverting the value just cited gives an average residence time of carbon dioxide under this extremely simplified alternate point of view. The average residence time, hypothetically, would thus be about 33 million years† per atmosphere. (That value is equivalent, under this crude viewpoint, to a “lifetime” of 13,000 years per CO2 at 400 ppm.) Contrast the two lifetime estimates with more detailed considerations by the IPCC (the Intergovernmental Panel on Climate Change). The latter group, recognizing that no single lifetime * T. F. Stocker et al., IPCC, 2013: Climate Change 2013 The Physical Science Basis, Cambridge University Press, Cambridge, New York (2013). † 0.03 atmospheres per million years, inverting the ratio gives a million years per 0.03 atm or 33 million years per atm.

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can be defined for CO2 because of the different rates of uptake by the various removal processes, suggests an average lifetime of CO2 in the air is between 5 and 200 years. Furthermore, the rate at which radiocarbon from atmospheric nuclear tests returned to background levels was mentioned in Chapter 5 as suggesting a mean residence time of about 15 years, reassuringly “within” the given range. Annual consumption of carbon dioxide by photosynthesis* is currently about 270 gigatons (billion tons, Gt) of carbon dioxide. This produces, reciprocally, 190 Gt of oxygen gas. The total oxygen content of the atmosphere is 1.2 quadrillion tons (1,200,000 Gt). From this, we can estimate that the cycle time for oxygen (dividing the latter value by the former value) is ≈6,000 years (since the total oxygen content has been essentially unchanged for much longer than 6,000 years). What we’re saying is, keeping the amount of oxygen in the atmosphere constant, as has been the case more or less for many millennia, the odds of any particular oxygen molecule being removed from the air amount to once every six millennia on the average. A similar approach for the carbon dioxide numbers for photosynthesis, 270 Gt CO2 removed yet a roughly constant 2,200 Gt CO2 in the atmosphere suggests an average turnover time of eight years for a photosynthetic carbon dioxide molecule.

CARBON DIOXIDE VARIATIONS Atop Hawaii, the carbon dioxide concentration at the 11,000 foot tall Mauna Loa peak was measured essentially every month beginning about 1955. This was the first direct measurement of CO2 in the atmosphere and was the instrumental brainchild of American chemist Charles Keeling of the Scripps Institute of Oceanography.

Charles Keeling

The appearance is now known as the Keeling curve. At this altitude in the islands, the air measured is very clean, monitoring was done continuously, wind corrections were easily accommodated, and instrument calibrations were checked every half hour. The average concentration increased steadily from 315 ppm to just over 400 ppm into the twenty-first century. That’s about a 27% change due mostly to increased combustion of fossil fuels worldwide. Superimposed on the gradual increase are very regular and discernible ≈1% annual fluctuations, 12-month cycles, that oscillate from about 3 ppm above the average to 3 ppm below the average. This is illustrated in the figure. The minima are in August; maxima are 6 months apart from minima. Minimum carbon * See Chapter 14.

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dioxide consumption and maximum air CO2 concentrations are in January, winter in Hawaii when plant growth has slowed or even ceased.

Furthermore, a cycle of the seasonal variations is clearly evident for both the Hawaii location (Mauna Loa) and for studies done in Point Barrow, Alaska. The variations are with respect to the average, shown as zero variation or 0 parts per million on the graph for both the years around 1960 and around 2010.*

At the South Pole, the same CO2 measurements were conducted over more than three decades. The results are displayed next (along with data from Samoa in the South Pacific and Point Barrow in Alaska above the Arctic Circle). The average atmospheric CO2 content again increased over the period studied. Annual oscillations are still clearly manifested, but their magnitude in the South Pole understandably is less by almost a factor of 3 because of the limited amount of photosynthesis associated with the seasons in Antarctica. There, positions of the maxima and minima have shifted one-half year because of season inversion in switching hemispheres from north to south. A careful look at the recent data indicates that although the average CO2 content was identical in Mauna Loa and the South Pole at the beginning of the studies in the 1950s (not shown), by the 1990s, Mauna Loa is slightly higher. This is consistent with the fact that more fossil fuel is used in the northern hemisphere and with the interpretation that exchange of carbon dioxide between north and south hemispheres is slow compared to the period over which the measurements have been made. * H. D. Graven et al., “Enhanced seasonal exchange of CO2 by northern ecosystems since 1960,” Science 341, 1085 (2013).

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The difference between peak and trough is referred to as the seasonal amplitude. There is some more discussion on this in the upcoming chapter on photosynthesis. All published measurements show averages, a fact that is inherently affected by time resolution. Poor time resolution can conceal useful details. The simplest example is for the Keeling curve figure where a display of annual averages would show a simple, structureless curve rising from about 315 ppm to about 410 ppm, missing all the seasonal amplitudes. Does that mean that monthly averaging reveals everything? Next, we display the 2016 measurements at Mauna Loa taken every hour (gray circles) for one month at the time that the carbon dioxide level had already exceeded the landmark 400 ppm reading in 2013. Note that daily averages (black circles), with even better time resolution than monthly averages, entirely miss the subtly highly variable structure in CO2 changes that are revealed with the data compiled each hour. Qualitatively, washing out of fluctuations when time resolution is imprecise must be kept in mind when interpreting archived proxies.

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HISTORICAL PRESCIENCE ON CARBON DIOXIDE Back in the 1820s, the French mathematician Joseph Fourier argued that the atmosphere acts like a greenhouse, letting in rays of sunlight, but retaining the rays returning upward from the ground, rays that we now know to be infrared light. His allusion to the greenhouse can actually be traced backward another 18 centuries to the Roman emperor, Tiberius, who had constructed enclosures made from the opaque mineral mica that would trap solar heat so that he could quench his craving for cucumbers during the off-season. In 1894, Swedish geologist Arvid Högbom published some methodical and insightful thoughts on carbon dioxide, or carbonic acid as it was referred to: Although it is not possible to obtain exact quantitative expressions for the reactions in nature by which carbonic acid is developed or consumed, nevertheless there are some factors, of which one may get an approximately true estimate, and from which certain conclusions that throw light on the question may be drawn. In the first place, it seems to be of importance to compare the quantity of carbonic acid now present in the air with the quantities that are being transformed. If the former is insignificant in comparison with the latter, then the probability for variations is wholly other than in the opposite case. The following calculation is also very instructive for the appreciation of the relation between the quantity of carbonic acid in the air and the quantities that are transformed. The world’s present production of coal reaches in round numbers 500 million tons per annum, or 1 ton per km.2 of the earth’s surface. Transformed into carbonic acid, this quantity would correspond to about a thousandth part of the carbonic acid in the atmosphere. It represents a layer of limestone of 0.003 millim. thickness over the whole globe, or 1.4 km.3 in cubic measure. This quantity of carbonic acid, which is supplied to the atmosphere chiefly by modern industry, may be regarded as completely compensating the quantity of carbonic acid that is consumed in the formation of limestone (or other mineral carbonates) by the weathering or decomposition of silicates. From the determination of the amounts of dissolved substances, especially carbonates, in a number of rivers in different countries and climates, and of the quantity of water flowing in these rivers and of their drainage-surface compared with the land-surface of the globe, it is estimated that the quantities of dissolved carbonates that are supplied to the ocean in the course of a year reach at most the bulk of 3 km.3. As it is also proved that the rivers the drainage regions of which consist of silicates convey very unimportant quantities of carbonates compared with those that flow through limestone regions, it is permissible to draw the conclusion, which is also strengthened by other reasons, that only an insignificant part of these 3 km.3 of carbonates is formed directly by decomposition of silicates. In other words, only an unimportant part of this quantity of carbonate of lime can be derived from the process of weathering in a year. Even though the number given were on account of inexact or uncertain assumptions erroneous to the extent of 50 per cent. or more, the comparison instituted is of very great interest, as it proves that the most important of all the processes by means of which carbonic acid has been removed from the atmosphere in all times, namely the chemical weathering of siliceous minerals, is of the same order of magnitude as a process of contrary effect, which is caused by the industrial development of our time, and which must be conceived of as being of a temporary nature. In comparison with the quantity of carbonic acid which is fixed in limestone (and other carbonates), the carbonic acid of the air vanishes. With regard to the thickness of sedimentary formations and the great part of them that is formed by limestone and other carbonates, it seems not improbable that the total quantity of carbonates would cover the whole earth’s surface to a height of hundreds of metres. If we assume 100 metres, – a number that may be inexact in a high degree, but probably is underestimated, – we find that about 25,000 times as much carbonic acid is fixed to lime in the sedimentary formations as exists free in the air. Every molecule of carbonic acid in this mass of limestone has, however, existed in and passed through the atmosphere in the course of time. Although we neglect all other factors which may have influenced the quantity of carbonic acid in the air, this number lends but very slight probability to the hypothesis, that this quantity should in former geological epochs have changed within limits which do not differ much from the present amount. As the process of weathering has consumed quantities of carbonic acid many thousand times greater than the amount now disposable in the air, and as this process from different geographical, climatological and other causes has in all likelihood proceeded with very different intensity at different epochs, the probability of important variations in the quantity of carbonic acid seems to be very great, even if we take into account the compensating

The Air Today

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processes which, as we shall see in what follows, are called forth as soon as, for one reason or another, the production or consumption of carbonic acid tends to displace the equilibrium to any considerable degree. One often hears the opinion expressed, that the quantity of carbonic acid in the air ought to have been very much greater formerly than now, and that the diminution should arise from the circumstance that carbonic acid has been taken from the air and stored in the earth’s crust in the form of coal and carbonates. In many cases this hypothetical diminution is ascribed only to the formation of coal, whilst the much more important formation of carbonates is wholly overlooked. This whole method of reasoning on a continuous diminution of the carbonic acid in the air loses all foundation in fact, notwithstanding that enormous quantities of carbonic acid in the course of time have been fixed in carbonates, if we consider more closely the processes by means of which carbonic acid has in all times been supplied to the atmosphere. From these we may well conclude that enormous variations have occurred, but not that the variation has always proceeded in the same direction. Carbonic acid is supplied to the atmosphere by the following processes:--(1) volcanic exhalations and geological phenomena connected therewith; (2) combustion of carbonaceous meteorites in the higher regions of the atmosphere; (3) combustion and decay of organic bodies; (4) decomposition of carbonates; (5) liberation of carbonic acid mechanically enclosed in minerals on the fracture or decomposition. The carbonic acid of the air is consumed chiefly by the following processes: –(6) formation of carbonates from silicates on weathering; and (7) the consumption of carbonic acid by vegetative processes. The ocean, too, plays an important role as a regulator of the quantity of carbonic acid in the air by means of the absorptive power of its water, which gives off carbonic acid as its temperature rises and absorbs it as it cools. The processes named under (4) and (5) are of little significance, so that they may be omitted. So too the processes (3) and (7), for the circulation of matter in the organic world goes on so rapidly that their variations cannot have any sensible influence. From this we must except periods in which great quantities or organisms were stored up in sedimentary formations and thus subtracted from the circulation, or in which such stored-up products were, as now, introduced anew into the circulation. The source of carbonic acid named in (2) is wholly incalculable. Thus the processes (1), (2), and (6) chiefly remain as balancing each other. As the enormous quantities of carbonic acid (representing a pressure of many atmospheres) that are now fixed in the limestone of the earth’s crust cannot be conceived to have existed in the air but as an insignificant fraction of the whole at any one time since organic life appeared on the globe, and since therefore the consumption through weathering and formation of carbonates must have been compensated by means of continuous supply, we must regard volcanic exhalations as the chief source of carbonic acid for the atmosphere. But this source has not flowed regularly and uniformly. Just as single volcanoes have their periods of variation with alternating relative rest and intense activity, in the same manner the globe as a whole seems in certain geological epochs to have exhibited a more violent and general volcanic activity, whilst other epochs have been marked by a comparative quiescence of the volcanic forces. It seems therefore probable that the quantity of carbonic acid in the air has undergone nearly simultaneous variations, or at least that this factor has had an important influence. If we pass the above-mentioned processes for consuming and producing carbonic acid under review, we find that they evidently do not stand in such a relation to or dependence on one another that any probability exists for the permanence of an equilibrium of the carbonic acid in the atmosphere. An increase or decrease of the supply continued during geological periods must, although it may not be important, conduce to remarkable alterations of the quantity of carbonic acid in the air, and there is no conceivable hindrance to imagining that this might in certain geological periods have been several times greater, or on the other hand considerably less, than now.

The above paragraphs were originally in Swedish. They were translated into English in 1896 by the renowned Swedish chemist and eventual Nobel laureate Svante Arrhenius. That year, Arrhenius published an article “On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground.” Arrhenius was sufficiently impressed with his colleague Högbom’s thoughts, that he included the above selections quoted directly into his own discourse. In his article, Arrhenius includes his ideas about the occurrence of glacial periods in relation to carbon dioxide atmospheric variations, something we will examine in Chapter 17.

7

Ye Olde Aire

This goodly frame, the earth, seems to me a sterile promontory; this most excellent canopy, the air, look you, this brave o’erhanging firmament, this majestical roof fretted with golden fire, why, it appeareth no other thing to me than a foul and pestilent congregation of vapours. W. SHAKESPEARE (Hamlet) As part of the carbon dioxide legacy, there is a need to address how to think about the air around us and also how that air composition was constituted over eons. We start with a reminder that early Earth (Chapter 2) was a very turbulent system. Volcanic activity, material bombardment, tectonic creeping of landmasses, continual subductions, churning magma, chemically reactive oceans, and violent weather raining down corrosive chemicals everywhere was pretty much the picture (innocently suggested in the epigraph by Shakespeare), although evidence of the accuracy of this picture of the young planet is extremely indirect. From the Oxford English Dictionary: Origin of air (the word)